Interview Preparation--------General Guide line

The most qualified job seekers need to prepare for job interviews. Interviewing is a learned skill, and there are no second chances to make a great first impression.

Google the Company!

Not being able to answer the question “What do you know about this company?” might just end your quest for employment, at least with this employer. Background information including company history, locations, divisions, and a mission statement are available in an “About Us” section on most company web sites. Review it ahead of time, then print it out and read it over just before your interview to refresh your memory.

Read the Job Description

It is imperative that you review the job description and the skills requested. Be able to articulate how your experience fits the need and how you can add value.

Introductions

It is important to communicate well with everyone you meet in your search for employment. It is, however, most important to positively connect with the person who might hire you. Shake hands, make eye contact, exude confidence, sit after the interviewer has sat down, engage the person you are speaking with, and you will let the interviewer know you are interested in the position. Insure you do this before you even answer any interview question.

Practice Good Nonverbal Communication

Insure you demonstrate confidence: standing straight, making eye contact and connecting with a good, firm handshake. That first impression can be a great beginning — or quick ending – to your interview.

Stay Calm

During the interview try to remain as calm as possible. Ask for clarification if you’re not sure what’s been asked and remember that it is perfectly acceptable to take a moment or two to frame your responses so you can be sure to fully answer the question.

Listen

From the very beginning of the interview, your interviewer is giving you information, either directly or indirectly. If you are not hearing it, you are missing a major opportunity. Good communication skills include listening and letting the person know you heard what was said. Observe your interviewer, and match that style and pace. Make sure you listen to the question and take a moment to gather your thoughts before you respond.

Communication Skills
Insure you speak slowly and clearly articulate your responses to questions. Do not allow your mouth to run ahead of your brain.

Answer the Questions

When an interviewer asks for an example of a time when you did something, he is seeking a sample of your past behavior. Do not be afraid to share projects that have gone awry, but be sure to follow it up with what you did to get the project back on track. If you fail to relate a specific example, you not only don’t answer the question, but you also miss an opportunity to prove your ability and talk about your skills.

Do Not Talk Too Much

There is nothing much worse than interviewing someone who goes on and on and on… The interviewer really doesn’t need to know your whole life story. Keep your answers succinct, to-the-point and focused and don’t ramble – simply answer the question.

Insure You Talk Enough

It’s really hard to communicate with someone who answers a question with a word or two. I remember a couple of interviews where I felt like I was pulling teeth to get any answers from the candidate. It wasn’t pleasant. So, even though you shouldn’t talk too much, you do want to be responsive and fully answer the question as best you can.

Badmouthing Past Employers

Your last boss was an idiot? Everyone in the company was a jerk? You hated your job and couldn’t wait to leave? Even if it’s true don’t say so. Stay Professional. It’s sometimes a smaller world than we think and you don’t know who your interviewer might know, including that boss who is an idiot… You also don’t want the interviewer to think that you might speak that way about his or her company if you leave on terms that aren’t the best.

Don’t Be Cocky

Attitude plays a key role in your interview success. There is a fine balance between confidence and arrogance. Even if you’re putting on a performance to demonstrate your ability, overconfidence is as bad, if not worse, as being too reserved.

Ask Questions

At the end of the interview you will usually be given a chance to ask any questions. When asked if they have any questions, most candidates answer, “No.” Wrong answer! It is extremely important to ask questions, not only do they allow the interviewee to acquire more information but they also demonstrate your interest in the position and company.
The best questions come from listening to what is asked during the interview and asking for additional information

Closing
When the Interview is Complete Insure you thank the manager for his time. In most cases the manager will advise you as to when he will make his decision. If he does not define that fact, it is perfectly acceptable to ask. Let the interviewer know that you think you would be a good fit for his or her team and look forward to hearing from him soon.

Oracle DBA Interview Questions and Answers---Internal Project selection on the corresponding company

ORACLE – ARCHITECTURE

What is difference between oracle SID and Oracle service name?

Oracle SID is the unique name that uniquely identifies your instance/database where as the service name is the TNS alias can be same or different as SID.

What are the steps to install oracle on Linux system? List two kernel parameter that effect oracle installation?

Initially set up disks and kernel parameters, then create oracle user and DBA group, and finally run installer to start the installation process. The SHMMAX & SHMMNI two kernel parameter required to set before installation process.

What are bind variables?

With bind variable in SQL, oracle can cache queries in a single time in the SQL cache area. This avoids a hard parse each time, which saves on various locking and latching resource we use to check object existence and so on.

What is the difference between data block/extent/segment?

A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.

What is the difference between PGA and UGA?

When you are running dedicated server then process information stored inside the process global area (PGA) and when you are using shared server then the process information stored inside user global area (UGA).

What is SGA? Define structure of shared pool component of SGA?

The system global area is a group of shared memory area that is dedicated to oracle instance. All oracle process uses the SGA to hold information. The SGA is used to store incoming data and internal control information that is needed by the database. You can control the SGA memory by setting the parameter db_cache_size, shared_pool_size and log_buffer.
Shared pool portion contain three major area:
Library cache (parse SQL statement, cursor information and execution plan),
data dictionary cache (contain cache, user account information, privilege user information, segments and extent information,data buffer cache for parallel execution message and control structure.

What is the difference between SMON and PMON processes?

SMON (System Monitor) performs recovery after instance failure, monitor temporary segments and extents; clean temp segment, coalesce free space. It is mandatory process of DB and starts by default.
PMON (Process Monitor) failed process resources. In shared server architecture monitor and restarts any failed dispatcher or server process. It is mandatory process of DB and starts by default.

What is a system change number (SCN)?

SCN is a value that is incremented whenever a dirty read occurs.
SCN is incremented whenever a deadlock occurs.
SCN is a value that keeps track of explicit locks.
SCN is a value that is incremented whenever database changes are made.

What is the main purpose of ‘CHECKPOINT’ in oracle database? How do you automatically force the oracle to perform a checkpoint?

A checkpoint is a database event, which synchronize the database blocks in memory with the datafiles on disk. It has two main purposes: To establish a data consistency and enable faster database Recovery.

The following are the parameter that will be used by DBA to adjust time or interval of how frequently its checkpoint should occur in database.
LOG_CHECKPOINT_TIMEOUT = 3600; # Every one hour
LOG_CHECKPOINT_INTERVAL = 1000; # number of OS blocks.

What happens when we fire SQL statement in Oracle?

First it will check the syntax and semantics in library cache, after that it will create execution plan.
If already data is in buffer cache it will directly return to the client.
If not it will fetch the data from datafiles and write to the database buffer cache after that it will send server and finally server send to the client.

What is the use of large pool, which case you need to set the large pool?

You need to set large pool if you are using: MTS (Multi thread server) and RMAN Backups. Large pool prevents RMAN & MTS from competing with other sub system for the same memory. RMAN uses the large pool for backup & restore when you set the DBWR_IO_SLAVES or BACKUP_TAPE_IO_SLAVES parameters to simulate asynchronous I/O. If neither of these parameters is enabled, then Oracle allocates backup buffers from local process memory rather than shared memory. Then there is no use of large pool.

What does database do during the mounting process?

While mounting the database oracle reads the data from controlfile which is used for verifying physical database files during sanity check. Background processes are started before mounting the database only.

What are logfile states?

“CURRENT” state means that redo records are currently being written to that group. It will be until a log switch occurs. At a time there can be only one redo group current.
If a redo group containing redo’s of a dirty buffer that redo group is said to be ‘ACTIVE’ state. As we know log file keep changes made to the data blocks then data blocks are modified in buffer cache (dirty blocks). These dirty blocks must be written to the disk (RAM to permanent media).
And when a redolog group contains no redo records belonging to a dirty buffer it is in an “INACTIVE” state. These inactive redolog can be overwritten.
One more state ‘UNUSED’ initially when you create new redo log group its log file is empty on that time it is unused. Later it can be any of the above mentioned state.

What is log switch?

The point at which oracle ends writing to one online redo log file and begins writing to another is called a log switch. Sometimes you can force the log switch.

ALTER SYSTEM SWITCH LOGFILE;

How to check Oracle database version?

SQL> Select * from v$version;

Explain Oracle Architecture?

Oracle Instance:
a means to access an Oracle database,always opens one and only one database and consists of memory structures and background process.

Oracle server:
a DBMS that provides an open, comprehensive, integrated approach to information management,Consists of an Instance and a database.

Oracle database:
a collection of data that is treated as a unit,Consists of Datafiles, Control files, Redo log files. (optional param file, passwd file, archived log)

Instance memory Structures:

System Global Area (SGA):
Allocated at instance startup, and is a fundamental component of an Oracle Instance.

SGA Memory structures:
Includes Shared Pool, Database Buffer Cache, Redo Log Buffer among others.

Shared Pool :
Consists of two key performance-related memory structures Library Cache and Data Dictionary Cache.

Library Cache:
Stores information about the most recently used SQL and PL/SQL statements and enables the sharing of commonly used statements.

Data Dictionary Cache :
Stores collection of the most recently used definitions in the database Includes db files, tables, indexes, columns etc. Improves perf. During the parse phase, the server process looks at the data dictionary for information to resolve object names and validate access.

Database Buffer Cache:
Stores copies of data blocks that have been retrieved from the datafiles. Everything done here.

Redo Log Buffer :
Records all changes made to the database data blocks, Primary purpose is recovery. Redo entries contain information to reconstruct or redo changes.

User process:
Started at the time a database User requests connection to the Oracle server. requests interaction with the Oracle server, does not interact directly with the Oracle server.

Server process:
Connects to the Oracle Instance and is Started when a user establishes a session.
fulfills calls generated and returns results.
Each server process has its own nonshared PGA when the process is started.
Server Process Parses and run SQL statements issued through the application, Reads necessary data blocks from datafiles on disk into the shared database buffers of the SGA, if the blocks are not already present in the SGA and Return results in such a way that the application can process the information.
In some situations when the application and Oracle Database operate on the same computer, it is possible to combine the user process and corresponding server process into a single process to reduce system overhead.

Program Global Area (PGA):
Memory area used by a single Oracle server process.
Allocated when the server process is started, deallocated when the process is terminated and used by only one process.
Used to process SQL statements and to hold logon and other session information.

Background processes:
Started when an Oracle Instance is started.
Background Processes Maintains and enforces relationships between physical and memory structures

There are two types of database processes:
1. Mandatory background processes
2. Optional background processes
Mandatory background processes:
– DBWn, PMON, CKPT, LGWR, SMON
Optional background processes:
– ARCn, LMDn, RECO, CJQ0, LMON, Snnn, Dnnn, Pnnn, LCKn, QMNn

DBWn writes when:
• Checkpoint occurs
• Dirty buffers reach threshold
• There are no free buffers
• Timeout occurs
• RAC ping request is made
• Tablespace OFFLINE
• Tablespace READ ONLY
• Table DROP or TRUNCATE
• Tablespace BEGIN BACKUP

Log Writer (LGWR) writes:
• At commit
• When 1/3rd full
• When there is 1 MB of redo
• Every 3 seconds
• Before DBWn writes

System Monitor (SMON) Responsibilities:
• Instance recovery
– Rolls forward changes in redo logs
– Opens database for user access
– Rolls back uncommitted transactions
• Coalesces free space
• Deallocates temporary segments.
Process Monitor (PMON) Cleans up after failed processes by:
• Rolling back the transaction
• Releasing locks
• Releasing other resources
• Restarting dead dispatchers

Checkpoint (CKPT) Responsible for:
• Signaling DBWn at checkpoints
• Updating datafile headers with checkpoint information
• Updating control files with checkpoint information
Archiver (ARCn)
• Optional background process
• Automatically archives online redo logs when ARCHIVELOG mode is set
• Preserves the record of all changes made to the database

Why do you run orainstRoot and ROOT.SH once you finalize the Installation?

orainstRoot.sh needs to be run to change the Permissions and groupname to 770 and to dba.
Root.sh (ORACLE_HOME) location needs to be run to create a ORATAB in /etc/oratab or /opt/var/oratab in Solaris and to copy dbhome, oraenv and coraenv to /usr/local/bin.
orainstRoot.sh
[root@oracle11g ~]# /u01/app/oraInventory/orainstRoot.sh
Changing permissions of /u01/app/oraInventory to 770.
Changing groupname of /u01/app/oraInventory to dba.
The execution of the script is complete
root.sh
[root@oracle11g ~]# /u01/app/oracle/product/11.1.0/db_1/root.sh
Running Oracle 11g root.sh script…
The following environment variables are set as:
ORACLE_OWNER= oracle
ORACLE_HOME= /u01/app/oracle/product/11.1.0/db_1
Enter the full pathname of the local bin directory: [/usr/local/bin]:
Copying dbhome to /usr/local/bin …
Copying oraenv to /usr/local/bin …
Copying coraenv to /usr/local/bin …
Creating /etc/oratab file…
Entries will be added to the /etc/oratab file as needed by
Database Configuration Assistant when a database is created
Finished running generic part of root.sh script.
Now product-specific root actions will be performed.
Finished product-specific root actions.
For Oracle installation on unix/linux, we will be prompted to run a script ‘root.sh’ from the oracle inventory directory.this script needs to run the first time only when any oracle product is installed on the server.
It creates the additional directories and sets appropriate ownership and permissions on files for root user.

Oracle Database 11g New Feature for DBAs?

1) Automatic Diagnostic Repository [ADR]
2) Database Replay
3) Automatic Memory Tuning
4) Case sensitive password
5) Virtual columns and indexes
6) Interval Partition and System Partition
7) The Result Cache
8) ADDM RAC Enhancements
9) SQL Plan Management and SQL Plan Baselines
10) SQL Access Advisor & Partition Advisor
11) SQL Query Repair Advisor
12) SQL Performance Analyzer (SPA) New
13) DBMS_STATS Enhancements
14) The Result Cache
15) Total Recall (Flashback Data Archive)
Note: The above are only top new features, there are other features as well introduced in 11g which will be included subsequently

What is the Difference Between Local Inventory and Global Inventory?
What is oraInventory ?

oraInventory is repository (directory) which store/records oracle software products & their oracle_homes location on a machine. This Inventory now a days in XML format and called as XML Inventory where as in past it used to be in binary format & called as binary Inventory.
There are basically two kind of inventories,
One is Local Inventory (also called as Oracle Home Inventory) and other is Global Inventory (also called as Central Inventory).

What is Global Inventory ?

Global Inventory holds information about Oracle Products on a Machine. These products can be various oracle components like database, oracle application server, collaboration suite, soa suite, forms & reports or discoverer server . This global Inventory location will be determined by file oraInst.loc in /etc (on Linux) or /var/opt/oracle (solaris). If you want to see list of oracle products on machine check for file inventory.xml under ContentsXML in oraInventory Please note if you have multiple global Inventory on machine check all oraInventory directories)
You will see entry like
HOME NAME=”ORA10g_HOME” LOC=”/u01/oracle/10.2.0/db” TYPE=”O” IDX=”1?/

What is Local Inventory ?

Inventory inside each Oracle Home is called as local Inventory or oracle_home Inventory. This Inventory holds information to that oracle_home only.

What is Oracle Home Inventory?

Oracle home inventory or local inventory is present inside each Oracle home. It only contains information relevant to a particular Oracle home. This file is located in the following location:
$ORACLE_HOME/inventory
It contains the following files and folders:
· Components File
· Home Properties File
· Other Folders

Can I have multiple Global Inventory on a machine ?

Quite common questions is that can you have multiple global Inventory and answer is YES you can have multiple global Inventory but if your upgrading or applying patch then change Inventory Pointer oraInst.loc to respective location. If you are following single global Inventory and if you wish to uninstall any software then remove it from Global Inventory as well.

What to do if my Global Inventory is corrupted ?
No need to worry if your global Inventory is corrupted, you can recreate global Inventory on machine using Universal Installer and attach already Installed oracle home by option
-attachHome
./runInstaller -silent -attachHome -invPtrLoc $location_to_oraInst.loc
ORACLE_HOME=”Oracle_Home_Location” ORACLE_HOME_NAME=”Oracle_Home_Name”
CLUSTER_NODES=”{}”

What is RESULT Cache?

11G Backgroung Processes?

The following process are added in 11g as new background processes.
1 dbrm DB resource manager
2 dia0 Diagnosability process
3 fbda Flashback data archiver process
4 vktm Virtual Timekeeper
5 w000 Space Management Co-ordination process
6 smc0 Space Manager process
NOTE : The above six are mandatory processes.
But 11g has 56 new processes added which can be queried using

If any one of these 6 mandatory background processes is killed/not running, the instance will be aborted ?

Background processes are started automatically when the instance is started.
Mandatory background processes are DBWn, LGWR, CKPT, SMON, PMON, and RECO. All other processes are optional, will be invoked if that particular feature is activated.
If any one of these 6 mandatory background processes is killed/not running, the instance will be aborted.
Any issues related to backgroud processes should be monitored and analyzed from the trace files generated and the alert log.

What is SGA_TARGET and SGA_MAX_SIZE ?

SGA_MAX_SIZE is the largest amount of memory that will be available for the SGA in the instance and it will be allocated from memory. You do not have to use it all, but it will be potentially wasted if you set it too high and don’t use it. It is not a dynamic parameter. Basically it gives you room for the Oracle instance to grow.
SGA_TARGET is actual memory in use by the current SGA. This parameter is dynamic and can be increased up to the value of SGA_MAX_SIZE.
SGA_MAX_SIZE and SGA_TARGET both are the parameter are used to change the SGA SIZE.
SGA_MAX_SIZE sets the maximum value for sga_target.
SGA_TAGET is 10G feature used to change the sga size dynamically .it specifies the total amount of SGA memory available to an instance.
this feature is called Automatic Shared Memory Management. With ASMM, the parameters java_pool_size, shared_pool_size, large_pool_size and db_cache_size are affected.

SGA_MAX_SIZE & SGA_TARGET

http://maxwellmiranda.wordpress.com/2009/09/17/sga_max_size-sga_targe/
SGA_MAX_SIZE sets the overall amount of memory the SGA can consume but is not dynamic.
The SGA_MAX_SIZE parameter is the max allowable size to resize the SGA Memory area parameters. If the SGA_TARGET is set to some value then the Automatic Shared Memory Management (ASMM) is enabled, the SGA_TARGET value can be adjusted up to the SGA_MAX_SIZE parameter, not more than SGA_MAX_SIZE parameter value.
I.e. If SGA_MAX_SIZE=4GB and SGA_TARGET=2GB, later period of time, if you want you can resize your SGA_TARGET parameter to the value of SGA_MAX_SIZE i.e. 4GB, you can’t resize the SGA_TARGET value to more than 4GB.
It is significant that SGA_TARGET includes the entire memory for the SGA, in contrast to earlier releases in which memory for the internal and fixed SGA was added to the sum of the configured SGA memory parameters. Thus, SGA_TARGET gives you precise control over the size of the shared memory region allocated by the database. If SGA_TARGET is set to a value greater than SGA_MAX_SIZE at startup, then the latter is bumped up to accommodate SGA_TARGET
Do not dynamically set or unset the SGA_TARGET parameter. This should be set only at startup.
SGA_TARGET is a database initialization parameter (introduced in Oracle 10g) that can be used for automatic SGA memory sizing.
SGA_TARGET provides the following:
§ Single parameter for total SGA size
§ Automatically sizes SGA components
§ Memory is transferred to where most needed
§ Uses workload information
§ Uses internal advisory predictions
§ STATISTICS_LEVEL must be set to TYPICAL
§ SGA_TARGET is dynamic
§ Can be increased till SGA_MAX_SIZE
§ Can be reduced till some component reaches minimum size
§ Change in value of SGA_TARGET affects only automatically sized components
If I keep SGA_TARGET =0 then what will happen ?
Disable automatic SGA tuning by setting sga_target=0
Disable ASMM by setting SGA_TARGET=0
http://www.orafaq.com/wiki/SGA_target
SGA_TARGET is a database initialization parameter (introduced in Oracle 10g) that can be used for automatic SGA memory sizing.
Default value 0 (SGA auto tuning is disabled)

What happens when you run ALTER DATABASE OPEN RESETLOGS ?

The current online redo logs are archived, the log sequence number is reset to 1, new database incarnation is created, and the online redo logs are given a new time stamp and SCN.
The reason to do the open the database with the resetlogs is that after doing an incomplete recovery , the data files and control files still don’t come to the same point of the redo log files. And as long as the database is not consistent within all the three file-data, redo and control, you can’t open the database. The resetlogs clause would reset the log sequence numbers within the log files and would start them from 0 thus enabling you to open the database but on the cost of losing all what was there in the redo log files.
In what scenarios open resetlogs required ?
An ALTER DATABASE OPEN RESETLOGS statement is required,
1.after incomplete recovery (Point in Time Recovery) or
2.recovery with a backup control file.
3. recovery with a control file recreated with the reset logs option.
http://onlineappsdba.com/index.php/2009/09/11/oracle-database-incarnation-open-resetlogs-scn/
http://web.njit.edu/info/limpid/DOC/backup.102/b14191/osrecov009.htm
Whenever you perform incomplete recovery or recovery with a backup control file, you must reset the online logs when you open the database. The new version of the reset database is called a new incarnation..

Difference between RESETLOGS and NORESETLOGS ?

http://oracleappstechnology.blogspot.in/2008/05/difference-between-resetlogs-and.html

After recover database operation, open the database with:

ALTER DATABASE OPEN [NO]RESETLOGS
NORESETLOGS:
The NORESETLOGS option does not clear the redo log files during startup and the online redo logs to be used for recovery. Only used in scenario where MANUAL RECOVERY is started, CANCEL is used, and then RECOVER DATABASE is started.
RESETLOGS:
CAUTION: Never use RESETLOGS unless necessary.
Once RESETLOGS is used then the redo log files cannot be used and any completed transactions in those redo logs are lost!!
Before using the RESETLOGS option take an offline backup of the database.

What is SCN (System Change Number) ?

The system change number (SCN) is an ever-increasing value that uniquely identifies a committed version of the database at a point in time. Every time a user commits a transaction Oracle records a new SCN in redo logs.
Oracle uses SCNs in control files datafile headers and redo records. Every redo log file has both a log sequence number and low and high SCN. The low SCN records the lowest SCN recorded in the log file while the high SCN records the highest SCN in the log file.

What is Database Incarnation ?

Database incarnation is effectively a new “version” of the database that happens when you reset the online redo logs using “alter database open resetlogs;”.
Database incarnation falls into following category Current, Parent, Ancestor and Sibling
i) Current Incarnation : The database incarnation in which the database is currently generating redo.
ii) Parent Incarnation : The database incarnation from which the current incarnation branched following an OPEN RESETLOGS operation.
iii) Ancestor Incarnation : The parent of the parent incarnation is an ancestor incarnation. Any parent of an ancestor incarnation is also an ancestor incarnation.
iv) Sibling Incarnation : Two incarnations that share a common ancestor are sibling incarnations if neither one is an ancestor of the other.

How to view Database Incarnation history of Database ?

Using SQL> select * from v$database_incarnation;
Using RMAN>LIST INCARNATION;
However, you can use the RESET DATABASE TO INCARNATION command to specify that SCNs are to be interpreted in the frame of reference of another incarnation.
•For example my current database INCARNATION is 3 and now I have used
FLASHBACK DATABASE TO SCN 3000;then SCN 3000 will be search in current incarnation which is 3. However if I want to get back to SCN 3000 of INCARNATION 2 then I have to use,

RMAN> RESET DATABASE TO INCARNATION 2;
RMAN> RECOVER DATABASE TO SCN 3000;
ORACLE – BACKUP AND RECOVERY
How would you decide your backup strategy and timing for backup?
In fact backup strategy is purely depends upon your organization business need.
If no downtime then database must be run on archivelog mode and you have to take frequently or daily backup.
If sufficient downtime is there and loss of data would not affect your business then you can run your database in noarchivelog mode and backup can be taken in-frequently or weekly or monthly.
In most of the case in an organization when no downtime then frequent inconsistent backup needed (daily backup), multiplex online redo log files (multiple copies), different location for redo log files, database must run in archivelog mode and dataguard can be implemented for extra bit of protection.

What is difference between Restoring and Recovery of database?

Restoring means copying the database object from the backup media to the destination where actually it is required where as recovery means to apply the database object copied earlier (roll forward) in order to bring the database into consistent state.

What is the difference between complete and incomplete recovery?

An incomplete database recovery is a recovery that it does not reach to the point of failure. The recovery can be either point of time or particular SCN or Particular archive log specially incase of missing archive log or redolog failure where as a complete recovery recovers to the point of failure possibly when having all archive log backup.

What is the benefit of running the DB in archivelog mode over no archivelog mode?

When a database is in no archivelog mode whenever log switch happens there will be a loss of some redoes log information in order to avoid this, redo logs must be archived. This can be achieved by configuring the database in archivelog mode.

If an oracle database is crashed?
How would you recover that transaction which is not in backup?

If the database is in archivelog we can recover that transaction otherwise we cannot recover that transaction which is not in backup.

What is the difference between HOTBACKUP and RMAN backup?

For hotbackup we have to put database in begin backup mode, then take backup where as RMAN would not put database in begin backup mode. RMAN is faster can perform incremental (changes only) backup, and does not place tablespace in hotbackup mode.

Can we use Same target database as Catalog database?

No, the recovery catalog should not reside in the target database (database to be backed up) because the database can not be recovered in the mounted state.

Incremental backup levels:
Level 0 – full backup that can be used for subsequent incrementals
RMAN> backup incremental level 0 database;
Differential Level 1–only the blocks that have changed since the last backup (whether it is level 0 or level 1)
RMAN> backup incremental level 1 differential database;
Cumulative Level 1 – all changes since the last level 0 incremental backup
RMAN> backup incremental level 1 cumulative database;
A full backup cannot be used for a cumulative level 1 backup.
A cumulative level 1 backup must be done on top of an incremental level 0 backup.

Why RMAN incremental backup fails even though full backup exists?

If you have taken the RMAN full backup using the command ‘Backup database’, where as a level 0 backup is physically identical to a full backup. The only difference is that the level 0 backup is recorded as an incremental backup in the RMAN repository so it can be used as the parent for a level 1 backup. Simply the ‘full backup without level 0’ can not be considered as a parent backup from which you can take level 1 backup.

Can we perform RMAN level 1 backup without level 0?

If no level 0 is available, then the behavior depends upon the compatibility mode setting (oracle version).
If the compatibility mode less than 10.0.0, RMAN generates a level 0 backup of files contents at the time of backup.
If the compatibility is greater than 10.0.0, RMAN copies all block changes since the file was created, and stores the results as level 1 backup.

How to put Manual/User managed backup in RMAN?

In case of recovery catalog, you can put by using catalog command:
RMAN> CATALOG START WITH ‘/oracle/backup.ctl’;

How to check RMAN version in oracle?

If you want to check RMAN catalog version then use the below query from SQL*plus
SQL> Select * from rcver;

What happens actually in case of instance Recovery?

While Oracle instance fails, Oracle performs an Instance Recovery when the associated database is being re-started. Instance recovery occurs in 2 steps:
Cache recovery: Changes being made to a database are recorded in the database buffer cache as well as redo log files simultaneously. When there are enough data in the database buffer cache, they are written to data files. If an Oracle instance fails before these data are written to data files, Oracle uses online redo log files to recover the lost data when the associated database is re-started. This process is called cache recovery.
Transaction recovery: When a transaction modifies data in a database (the before image of the modified data is stored in an undo segment which is used to restore the original values in case the transaction is rolled back). At the time of an instance failure, the database may have uncommitted transactions. It is possible that changes made by these uncommitted transactions have gotten saved in data files. To maintain read consistency, Oracle rolls back all uncommitted transactions when the associated database is re-started. Oracle uses the undo data stored in undo segments to accomplish this. This process is called transaction recovery.

ORACLE PATCHING,CLONING & UPGRADE

When you moved oracle binary files from one ORACLE_HOME server to another server then which oracle utility will be used to make this new ORACLE_HOME usable?

Relink all.

In which months oracle release CPU patches?

JAN, APR, JUL, OCT

When we applying single Patch, can you use opatch utility?
Yes, you can use Opatch incase of single patch. The only type of patch that cannot be used with OPatch is a patchset.

Is it possible to apply OPATCH without downtime?

As you know for apply patch your database and listener must be down. When you apply OPTACH it will update your current ORACLE_HOME. Thus coming to your question to the point in fact it is not possible without or zero downtime in case of single instance but in RAC you can Apply Opatch without downtime as there will be more separate ORACLE_HOME and more separate instances (running once instance on each ORACLE_HOME).

You have collection of patch (nearly 100 patches) or patchset. How can you apply only one patch from it?

With Napply itself (by providing patch location and specific patch id) you can apply only one patch from a collection of extracted patch. For more information check the opatch util NApply –help. It will give you clear picture.

For Example:
opatch util napply -id 9 -skip_subset -skip_duplicate
This will apply only the patch id 9 from the patch location and will skip duplicate and subset of patch installed in your ORACLE_HOME.

If both CPU and PSU are available for given version which one, you will prefer to apply?

From the above discussion it is clear once you apply the PSU then the recommended way is to apply the next PSU only. In fact, no need to apply CPU on the top of PSU as PSU contain CPU (If you apply CPU over PSU will considered you are trying to rollback the PSU and will require more effort in fact). So if you have not decided or applied any of the patches then, I will suggest you to go to use PSU patches. For more details refer: Oracle Products [ID 1430923.1], ID 1446582.1

PSU is superset of CPU then why someone choose to apply a CPU rather than a PSU?

CPUs are smaller and more focused than PSU and mostly deal with security issues. It seems to be theoretically more consecutive approach and can cause less trouble than PSU as it has less code changing in it. Thus any one who is concerned only with security fixes and not functionality fixes, CPU may be good approach.

How to Download Patches, Patchset or Opatch from metalink?

If you are using latest support.oracle.com then after login to metalink Dashboard
– Click on “Patches & Updates” tab
– On the left sidebar click on “Latest Patchsets” under “Oracle Server/Tools”.
– A new window will appear.
– Just mouseover on your product in the “Latest Oracle Server/Tools Patchsets” page.
– Corresponding oracle platform version will appear. Then simply choose the patchset version and click on that.
– You will go the download page. From the download page you can also change your platform and patchset version.

REFERENCES:
http://docs.oracle.com/cd/E11857_01/em.111/e12255/e_oui_appendix.htm
Oracle® Universal Installer and OPatch User’s Guide
11g Release 2 (11.2) for Windows and UNIX
Part Number E12255-11

What is the recent Patch applied?

What is OPatch?

How to Apply Opatch in Oracle?

1. You MUST read the Readme.txt file included in opatch file, look for any prereq. steps/ post installation steps or and DB related changes. Also, make sure that you have the correct opatch version required by this patch.
2.Make sure you have a good backup of database.
3. Make a note of all Invalid objects in the database prior to the patch.
4. Shutdown All the Oracle Processes running from that Oracle Home , including the Listener and Database instance, Management agent etc.
5. You MUST Backup your oracle Home and Inventory
tar -cvf $ORACLE_HOME $ORACLE_HOME/oraInventory | gzip > Backup_Software_Version.tar.gz
6. Unzip the patch in $ORACLE_HOME/patches
7. cd to the patch direcory and do opatch -apply to apply the patch.
8. Read the output/log file to make sure there were no errors.

Patching Oracle Software with OPatch ?

opatch napply -skip_subset -skip_duplicate
OPatch skips duplicate patches and subset patches (patches under that are subsets of patches installed in the Oracle home).

What is Opactch in Oracle?

OPATCH Utility (Oracle RDBMS Patching)

1. Download the required Patch from Metalink based on OS Bit Version and DB Version.
2. Need to down the database before applying patch.
3. Unzip and Apply the Patch using ”opatch apply” command.On successfully applied of patch you will see successful message “OPatch succeeded.“, Crosscheck your patch is applied by using “opatch lsinventory” command .
4. Each patch has a unique ID, the command to rollback a patch is “opatch rollback -id ” command.On successfully applied of patch you will see successful message “OPatch succeeded.“, Crosscheck your patch is applied by using “opatch lsinventory” command .
5. Patch file format will be like, “p__.zip”
6. We can check the opatch version using “opatch -version” command.
7. Generally, takes 2 minutes to apply a patch.
8. To get latest Opatch version download “patch 6880880 – latest opatch tool”, it contains OPatch directory.
9. Contents of downloaded patches will be like “etc,files directories and a README file”
10. Log file for Opatch utility can be found at $ORACLE_HOME/cfgtoollogs/opatch
11. OPatch also maintains an index of the commands executed with OPatch and the log files associated with it in the history.txt file located in the /cfgtoollogs/opatch directory.
12. Starting with the 11.2.0.2 patch set, Oracle Database patch sets are full installations of the Oracle Database software. This means that you do not need to install Oracle Database 11g Release 2 (11.2.0.1) before installing Oracle Database 11g Release 2 (11.2.0.2).
13. Direct upgrade to Oracle 10g is only supported if your database is running one of the following releases: 8.0.6, 8.1.7, 9.0.1, or 9.2.0. If not, you will have to upgrade the database to one of these releases or use a different upgrade option (like export/ import).
14.Direct upgrades to 11g are possible from existing databases with versions 9.2.0.4+, 10.1.0.2+ or 10.2.0.1+. Upgrades from other versions are supported only via intermediate upgrades to a supported upgrade version.

http://avdeo.com/2008/08/19/opatch-utility-oracle-rdbms-patching/

Oracle version 10.2.0.4.0 what does each number refers to?
Oracle version number refers:
10 – Major database release number
2 – Database Maintenance release number
0 – Application server release number
4 – Component Specific release number
0 – Platform specific release number
ORACLE – ASM
What is ASM in Oracle?

Oracle ASM is Oracle’s volume manager specially designed for Oracle database data. It is available since Oracle database version 10g and many improvements have been made in versions 11g release 1 and 2.

ASM offers support for Oracle RAC clusters without the requirement to install 3rd party software, such as cluster aware volume managers or filesystems.

ASM is shipped as part of the database server software (Enterprise and Standard editions) and does not cost extra money to run.

ASM simplifies administration of Oracle related files by allowing the administrator to reference disk groups
rather than individual disks and files, which are managed by ASM.

The ASM functionality is an extention of the Oracle Managed Files (OMF) functionality that also includes striping and mirroring to provide balanced and secure storage. The new ASM functionality can be used in combination with existing raw and cooked file systems, along with OMF and manually managed files.

Advantages of ASM in Oracle?
Provides automatic load balancing over all the available disks, thus reducing hot spots in the file system

Prevents fragmentation of disks, so you don’t need to manually relocate data to tune I/O performance

Adding disks is straight forward – ASM automatically performs online disk reorganization when you add or remove storage

Uses redundancy features available in intelligent storage arrays

The storage system can store all types of database files

Using disk group makes configuration easier, as files are placed into disk groups

ASM provides stripping and mirroring (fine and coarse gain – see below)

ASM and non-ASM oracle files can coexist

Striping—ASM spreads data evenly across all disks in a disk group to optimize performance and utilization. This even distribution of database files eliminates the need for regular monitoring and I/O performance tuning.

For example, if there are six disks in a disk group, pieces of each ASM file are written to all six disks. These pieces come in 1 MB chunks known as extents. When a database file is created, it is striped (divided into extents and distributed) across the six disks, and allocated disk space on all six disks grows evenly. When reading the file, file extents are read from all six disks in parallel, greatly increasing performance.

Mirroring—ASM can increase availability by optionally mirroring any file. ASM mirrors at the file level, unlike operating system mirroring, which mirrors at the disk level. Mirroring means keeping redundant copies, or mirrored copies, of each extent of the file, to help avoid data loss caused by disk failures. The mirrored copy of each file extent is always kept on a different disk from the original copy. If a disk fails, ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.

ASM supports 2-way mirroring, where each file extent gets one mirrored copy, and 3-way mirroring, where each file extent gets two mirrored copies.

Online storage reconfiguration and dynamic rebalancing—ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, ASM automatically redistributes the data so that it is evenly spread across all disks in the disk group, including the new disk. This redistribution is known as rebalancing. It is done in the background and with minimal impact to database performance. When you request to remove a disk, ASM first rebalances by evenly relocating all file extents from the disk being removed to the other disks in the disk group.

Managed file creation and deletion—ASM further reduces administration tasks by enabling files stored in ASM disk groups to be Oracle-managed files. ASM automatically assigns filenames when files are created, and automatically deletes files when they are no longer needed.

What is ASM instance in Oracle?

The ASM functionality is controlled by an ASM instance. This is not a full database instance, just the memory structures and as such is very small and lightweight.

Characteristics of Oracle ASM instance

1. do not have controlfile and datafiles, do not have online redo logs

2. do have init.ora and a passwordfile
3. for connecting remotely, create passwordfile and set following in init.ora
remote_login_passwordfile=exclusive

create a password file:
$ORACLE_HOME/bin/orapwd file=orapw+ASM1 password=yourpw entries=10
4. ASM instance can not be in open status as there are not datafiles. Can be in mount (although
there is no controlfile) and nomount status. When in mount status, database can use the
diskgroup. The mount status actually means mount disk groups.

What are ASM Background Processes in Oracle?

Both an Oracle ASM instance and an Oracle Database instance are built on the same technology. Like a database instance, an Oracle ASM instance has memory structures (System Global Area) and background processes. Besides, Oracle ASM has a minimal performance impact on a server. Rather than mounting a database, Oracle ASM instances mount disk groups to make Oracle ASM files available to database instances.

There are at least two new background processes added for an ASM instance:

ASM Instance Background Processes:

ARBx (ASM) Rebalance working processARBn performs the actual rebalance data extent movements in an Automatic Storage Management instance. There can be many of these processes running at a time, named ARB0, ARB1, and so on.These processes are managed by the RBAL process. The number of ARBx processes invoked is directly influenced by the asm_power_limit parameter.

RBAL (Re-balancer) RBAL runs in both database and ASM instances. In the database instance, it does a global open of ASM disks. In an ASM instance, it also coordinates rebalance activity for disk groups.RBAL, which coordinates rebalance activities
for disk resources controlled by ASM.

Database Instance ASM Background Processes:
In the database instances, there are three background process to support ASM, namely:

ASMB, this process contact CSS using the group name and acquires the associated ASM connect string. The connect string is subsequently used to connect to the ASM instance.

RBAL, which performs global opens on all disks in the disk group.A global open means that more than one database instance can be accessing the ASM disks at a time.

O00x, a group slave processes, with a numeric sequence starting at 000.

What are the components of components of ASM are disk groups?
The main components of ASM are disk groups, each of which comprise of several physical disks that are controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are know as ASM files. The locations and names for the files are controlled by ASM, but user-friendly aliases and directory structures can be defined for ease of reference.

Failure groups are defined within a disk group to support the required level of redundancy. For two-way mirroring you would expect a disk group to contain two failure groups so individual files are written to two locations.

What are ASM instance initialization parameters?

INSTANCE_TYPE – Set to ASM or RDBMS depending on the instance type. The default is RDBMS.

DB_UNIQUE_NAME – Specifies a globally unique name for the database. This defaults to +ASM but must be altered if you intend to run multiple ASM instances.

ASM_POWER_LIMIT -The maximum power for a rebalancing operation on an ASM instance. The valid values range from 1 to 11, with 1 being the default. The higher the limit the more resources are allocated resulting in faster rebalancing operations. This value is also used as the default when the POWER clause is omitted from a rebalance operation.

ASM_DISKGROUPS – The list of disk groups that should be mounted by an ASM instance during instance startup, or by the ALTER DISKGROUP ALL MOUNT statement. ASM configuration changes are automatically reflected in this parameter.

ASM_DISKSTRING – Specifies a value that can be used to limit the disks considered for discovery. Altering the default value may improve the speed of disk group mount time and the speed of adding a disk to a disk group. Changing the parameter to a value which prevents the discovery of already mounted disks results in an error. The default value is NULL allowing all suitable disks to be considered.

Advantages of ASM in Oracle?

Provides automatic load balancing over all the available disks, thus reducing hot spots in the file system

Prevents fragmentation of disks, so you don’t need to manually relocate data to tune I/O performance

Adding disks is straight forward – ASM automatically performs online disk reorganization when you add or remove storage

Uses redundancy features available in intelligent storage arrays

The storage system can store all types of database files

Using disk group makes configuration easier, as files are placed into disk groups

ASM provides stripping and mirroring (fine and coarse gain – see below)

ASM and non-ASM oracle files can coexist

Striping—ASM spreads data evenly across all disks in a disk group to optimize performance and utilization. This even distribution of database files eliminates the need for regular monitoring and I/O performance tuning.

For example, if there are six disks in a disk group, pieces of each ASM file are written to all six disks. These pieces come in 1 MB chunks known as extents. When a database file is created, it is striped (divided into extents and distributed) across the six disks, and allocated disk space on all six disks grows evenly. When reading the file, file extents are read from all six disks in parallel, greatly increasing performance.

Mirroring – ASM can increase availability by optionally mirroring any file. ASM mirrors at the file level, unlike operating system mirroring, which mirrors at the disk level. Mirroring means keeping redundant copies, or mirrored copies, of each extent of the file, to help avoid data loss caused by disk failures. The mirrored copy of each file extent is always kept on a different disk from the original copy. If a disk fails, ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.

ASM supports 2-way mirroring, where each file extent gets one mirrored copy, and 3-way mirroring, where each file extent gets two mirrored copies.

Online storage reconfiguration and dynamic rebalancing—ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, ASM automatically redistributes the data so that it is evenly spread across all disks in the disk group, including the new disk. This redistribution is known as rebalancing. It is done in the background and with minimal impact to database performance. When you request to remove a disk, ASM first rebalances by evenly relocating all file extents from the disk being removed to the other disks in the disk group.

Managed file creation and deletion—ASM further reduces administration tasks by enabling files stored in ASM disk groups to be Oracle-managed files. ASM automatically assigns filenames when files are created, and automatically deletes files when they are no longer needed.

Why should we use separate ASM home?

ASM should be installed separately from the database software in its own ORACLE_HOME directory. This will allow you the flexibility to patch and upgrade ASM and the database software independently.

How many ASM instances should one have?

Several databases can share a single ASM instance. So, although one can create multiple ASM instances on a single system, normal configurations should have one and only one ASM instance per system.

For clustered systems, create one ASM instance per node (called +ASM1, +ASM2, etc).

How many diskgroups should one have?

Generally speaking one should have only one disk group for all database files – and, optionally a second for recovery files (see FRA).

Data with different storage characteristics should be stored in different disk groups. Each disk group can have different redundancy (mirroring) settings (high, normal and external), different fail-groups, etc. However, it is generally not necessary to create many disk groups with the same storage characteristics (i.e. +DATA1, +DATA2, etc. all on the same type of disks).

To get started, create 2 disk groups – one for data and one for recovery files. Here is an example:

CREATE DISKGROUP data EXTERNAL REDUNDANCY DISK ‘/dev/d1′, ‘/dev/d2′, ‘/dev/d3′, ….;
CREATE DISKGROUP recover EXTERNAL REDUNDANCY DISK ‘/dev/d10′, ‘/dev/d11′, ‘/dev/d12′, ….;

Here is an example how you can enable automatic file management with such a setup:

ALTER SYSTEM SET db_create_file_dest = ‘+DATA’ SCOPE=SPFILE;
ALTER SYSTEM SET db_recovery_file_dest = ‘+RECOVER’ SCOPE=SPFILE;

You may also decide to introduce additional disk groups – for example, if you decide to put historic data on low cost disks, or if you want ASM to mirror critical data across 2 storage cabinets.

What is ASM Rebalancing?

The rebalancing speed is controlled by the ASM_POWER_LIMIT initialization parameter. Setting it to 0 will disable disk rebalancing.

ALTER DISKGROUP data REBALANCE POWER 11;

What happens when an Oracle ASM diskgroup is created?

When an ASM diskgroup is created, a hierarchialfilesystem structure is created.

How does this filesystem structure appear?

Oracle ASM diskgroup’sfilesystem structure is similar to UNIX filesystem hierarchy or Windows filesystem hierarchy.

Where are the Oracle ASM files stored?

Oracle ASM files are stored within the Oracle ASM diskgroup. If we dig into internals, oracle ASM files are stored within the Oracle ASM filesystem structures.

How are the Oracle ASM files stored within the Oracle ASM filesystem structure?

Oralce ASM files are stored within the Oracle ASM filesystem structures as objects that RDBMS instances/Oracle database instance access. RDBMS/Oracle instance treats the Oracle ASM files as standard filesystem files.

What are the Oracle ASM files that are stored within the Oracle ASM file hierarchy?

Files stored in Oracle ASM diskgroup/Oracl ASM filestructures include:
1) Datafile
2) Controlfiles
3) Server Parameter Files(SPFILE)
4) Redo Log files

What happens when you create a file/database file in ASM?What commands do you use to create database files?

Some common commands used for creating database files are :
1) Create tabespace
2) Add Datafile
3) Add Logfile
For example,
SQL> CREATE TABLESPACE TS1 DATAFILE ‘+DATA1′ SIZE 10GB;
Above command creates a datafile in DATA1 diskgroup

How can you access a databasefile in ASM diskgroup under RDBMS?

Once the ASM file is created in ASM diskgroup, a filename is generated. This file is now visible to the user via the standard RDBMS view V$DATAFILE.

What will be the syntax of ASM filenames?

ASM filename syntax is as follows:
+diskgroup_name/database_name/database_file_type/tag_name.file_number.incarnation
where,
+diskgroup_name – Name of the diskgroup that contains this file
database_name – Name of the database that contains this file
datafile – Can be one among 20 different ASM file types
tag_name – corresponds to tablespace name for datafiles, groupnumber for redo log files
file_number – file_number in ASM instance is used to correlate filenames in database instance
incarnation_number – It is derived from the timestamp. IT is used to provide uniqueness

What is an incarnation number?

An incarnation number is a part of ASM filename syntax. It is derived from the timestamp. Once the file is created, its incarnation number doesnot change.

What is the use of an incarnation number in Oracle ASM filename?

Incarnation number distinguishes between a new file that has been created using the same file number and another file that has been deleted

ASM’s SPFile will be residing inside ASM itself. This could be found out in number of ways, looking at the alert log of ASM when ASM starts
Machine: x86_64
Using parameter settings in server-side spfile +DATA/asm/asmparameterfile/registry.253.766260991
System parameters with non-default values:
large_pool_size = 12M
instance_type = “asm”
remote_login_passwordfile= “EXCLUSIVE”
asm_diskgroups = “FLASH”
asm_diskgroups = “DATA”
asm_power_limit = 1
diagnostic_dest = “/opt/app/oracle”
Or using the asmcmd’s spget command which shows the spfile location registered with GnP profile
ASMCMD> spget
+DATA/asm/asmparameterfile/registry.253.766260991
ORACLE – RAC

What is RAC? What is the benefit of RAC over single instance database?

In Real Application Clusters environments, all nodes concurrently execute transactions against the same database. Real Application Clusters coordinates each node’s access to the shared data to provide consistency and integrity.

Benefits:
Improve response time
Improve throughput
High availability
Transparency

What is Oracle RAC One Node?

Oracle RAC one Node is a single instance running on one node of the cluster while the 2nd node is in cold standby mode. If the instance fails for some reason then RAC one node detect it and restart the instance on the same node or the instance is relocate to the 2nd node incase there is failure or fault in 1st node. The benefit of this feature is that it provides a cold failover solution and it automates the instance relocation without any downtime and does not need a manual intervention. Oracle introduced this feature with the release of 11gR2 (available with Enterprise Edition).

Real Application Clusters
Oracle RAC is a cluster database with a shared cache architecture that overcomes the limitations of traditional shared-nothing and shared-disk approaches to provide a highly scalable and available database solution for all your business applications. Oracle RAC provides the foundation for enterprise grid computing.

Oracle’s Real Application Clusters (RAC) option supports the transparent deployment of a single database across a cluster of servers, providing fault tolerance from hardware failures or planned outages. Oracle RAC running on clusters provides Oracle’s highest level of capability in terms of availability, scalability, and low-cost computing.

One DB opened by multipe instances so the the db ll be Highly Available if an instance crashes.
Cluster Software. Oracles Clusterware or products like Veritas Volume Manager are required to provide the cluster support and allow each node to know which nodes belong to the cluster and are available and with Oracle Cluterware to know which nodes have failed and to eject then from the cluster, so that errors on that node can be cleared.

Oracle Clusterware has two key components Cluster Registry OCR and Voting Disk.

The cluster registry holds all information about nodes, instances, services and ASM storage if used, it also contains state information ie they are available and up or similar.

The voting disk is used to determine if a node has failed, i.e. become separated from the majority. If a node is deemed to no longer belong to the majority then it is forcibly rebooted and will after the reboot add itself again the the surviving cluster nodes.

Advantages of RAC (Real Application Clusters)

Reliability – if one node fails, the database won’t fail
Availability – nodes can be added or replaced without having to shutdown the database
Scalability – more nodes can be added to the cluster as the workload increases

What is a virtual IP address or VIP?
A virtual IP address or VIP is an alternate IP address that the client connections use instead of the standard public IP address. To configure VIP address, we need to reserve a spare IP address for each node, and the IP addresses must use the same subnet as the public network.

What is the use of VIP?

If a node fails, then the node’s VIP address fails over to another node on which the VIP address can accept TCP connections but it cannot accept Oracle connections.
Give situations under which VIP address failover happens:-
VIP addresses failover happens when the node on which the VIP address runs fails, all interfaces for the VIP address fails, all interfaces for the VIP address are disconnected from the network.
Using virtual IP we can save our TCP/IP timeout problem because Oracle notification service maintains communication between each nodes and listeners.

What is the significance of VIP address failover?

When a VIP address failover happens, Clients that attempt to connect to the VIP address receive a rapid connection refused error .They don’t have to wait for TCP connection timeout messages.

What is voting disk?

Voting Disk is a file that sits in the shared storage area and must be accessible by all nodes in the cluster. All nodes in the cluster registers their heart-beat information in the voting disk, so as to confirm that they are all operational. If heart-beat information of any node in the voting disk is not available that node will be evicted from the cluster. The CSS (Cluster Synchronization Service) daemon in the clusterware maintains the heart beat of all nodes to the voting disk. When any node is not able to send heartbeat to voting disk, then it will reboot itself, thus help avoiding the split-brain syndrome.

For high availability, Oracle recommends that you have a minimum of three or odd number (3 or greater) of votingdisks.

Voting Disk – is file that resides on shared storage and Manages cluster members. Voting disk reassigns cluster ownership between the nodes in case of failure.

The Voting Disk Files are used by Oracle Clusterware to determine which nodes are currently members of the cluster. The voting disk files are also used in concert with other Cluster components such as CRS to maintain the clusters integrity.

Oracle Database 11g Release 2 provides the ability to store the voting disks in ASM along with the OCR. Oracle Clusterware can access the OCR and the voting disks present in ASM even if the ASM instance is down. As a result CSS can continue to maintain the Oracle cluster even if the ASM instance has failed.

How many voting disks are you maintaining ?

http://www.toadworld.com/KNOWLEDGE/KnowledgeXpertforOracle/tabid/648/TopicID/RACR2ARC6/Default.aspx

By default Oracle will create 3 voting disk files in ASM.

Oracle expects that you will configure at least 3 voting disks for redundancy purposes. You should always configure an odd number of voting disks >= 3. This is because loss of more than half your voting disks will cause the entire cluster to fail.

You should plan on allocating 280MB for each voting disk file. For example, if you are using ASM and external redundancy then you will need to allocate 280MB of disk for the voting disk. If you are using ASM and normal redundancy you will need 560MB.

Why we need to keep odd number of voting disks ?

Oracle expects that you will configure at least 3 voting disks for redundancy purposes. You should always configure an odd number of voting disks >= 3. This is because loss of more than half your voting disks will cause the entire cluster to fail.

What are Oracle RAC software components?

Oracle RAC is composed of two or more database instances. They are composed of Memory structures and background processes same as the single instance database.Oracle RAC instances use two processes GES(Global Enqueue Service), GCS(Global Cache Service) that enable cache fusion.Oracle RAC instances are composed of following background processes:
ACMS – Atomic Controlfile to Memory Service (ACMS)
GTX0-j – Global Transaction Process
LMON – Global Enqueue Service Monitor
LMD – Global Enqueue Service Daemon
LMS – Global Cache Service Process
LCK0 – Instance Enqueue Process
RMSn – Oracle RAC Management Processes (RMSn)
RSMN – Remote Slave Monitor

What are Oracle Clusterware processes for 10g ?

Cluster Synchronization Services (ocssd) — Manages cluster node membership and runs as the oracle user; failure of this process results in cluster restart.
Cluster Ready Services (crsd) — The crs process manages cluster resources (which could be a database, an instance, a service, a Listener, a virtual IP (VIP) address, an application process, and so on) based on the resource’s configuration information that is stored in the OCR. This includes start, stop, monitor and failover operations. This process runs as the root user
Event manager daemon (evmd) —A background process that publishes events that crs creates.
Process Monitor Daemon (OPROCD) —This process monitor the cluster and provide I/O fencing. OPROCD performs its check, stops running, and if the wake up is beyond the expected time, then OPROCD resets the processor and reboots the node. An OPROCD failure results in Oracle Clusterware restarting the node. OPROCD uses the hangcheck timer on Linux platforms.
RACG (racgmain, racgimon) —Extends clusterware to support Oracle-specific requirements and complex resources. Runs server callout scripts when FAN events occur.

What are Oracle database background processes specific to RAC?

LMS—Global Cache Service Process
LMD—Global Enqueue Service Daemon
LMON—Global Enqueue Service Monitor
LCK0—Instance Enqueue Process
Oracle RAC instances use two processes, the Global Cache Service (GCS) and the Global Enqueue Service (GES). The GCS and GES maintain records of the statuses of each data file and each cached block using a Global Resource Directory (GRD). The GRD contents are distributed across all of the active instances.

What is Cache Fusion?

Transfor of data across instances through private interconnect is called cachefusion.Oracle RAC is composed of two or more instances. When a block of data is read from datafile by an instance within the cluster and another instance is in need of the same block,it is easy to get the block image from the insatnce which has the block in its SGA rather than reading from the disk. To enable inter instance communication Oracle RAC makes use of interconnects. The Global Enqueue Service(GES) monitors and Instance enqueue process manages the cahce fusion

What is SCAN? (11gR2 feature)

Single Client Access Name (SCAN) is s a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access an Oracle Database running in a cluster. The benefit is clients using SCAN do not need to change if you add or remove nodes in the cluster.

SCAN provides a single domain name via (DNS), allowing and-users to address a RAC cluster as-if it were a single IP address. SCAN works by replacing a hostname or IP list with virtual IP addresses (VIP).

Single client access name (SCAN) is meant to facilitate single name for all Oracle clients to connect to the cluster database, irrespective of number of nodes and node location. Until now, we have to keep adding multiple address records in all clients tnsnames.ora, when a new node gets added to or deleted from the cluster.

Single Client Access Name (SCAN) eliminates the need to change TNSNAMES entry when nodes are added to or removed from the Cluster. RAC instances register to SCAN listeners as remote listeners. Oracle recommends assigning 3 addresses to SCAN, which will create 3 SCAN listeners, though the cluster has got dozens of nodes.. SCAN is a domain name registered to at least one and up to three IP addresses, either in DNS (Domain Name Service) or GNS (Grid Naming Service). The SCAN must resolve to at least one address on the public network. For high availability and scalability, Oracle recommends configuring the SCAN to resolve to three addresses.
http://www.freeoraclehelp.com/2011/12/scan-setup-for-oracle-11g-release211gr2.html

What are SCAN components in a cluster?

1.SCAN Name
2.SCAN IPs (3)
3.SCAN Listeners (3)

What is FAN?

Fast application Notification as it abbreviates to FAN relates to the events related to instances,services and nodes.This is a notification mechanism that Oracle RAc uses to notify other processes about the configuration and service level information that includes service status changes such as,UP or DOWN events.Applications can respond to FAN events and take immediate action.

What is TAF?

TAF (Transparent Application Failover) is a configuration that allows session fail-over between different nodes of a RAC database cluster.
Transparent Application Failover (TAF). If a communication link failure occurs after a connection is established, the connection fails over to another active node. Any disrupted transactions are rolled back, and session properties and server-side program variables are lost. In some cases, if the statement executing at the time of the failover is a Select statement, that statement may be automatically re-executed on the new connection with the cursor positioned on the row on which it was positioned prior to the failover.

After an Oracle RAC node crashes—usually from a hardware failure—all new application transactions are automatically rerouted to a specified backup node. The challenge in rerouting is to not lose transactions that were “in flight” at the exact moment of the crash. One of the requirements of continuous availability is the ability to restart in-flight application transactions, allowing a failed node to resume processing on another server without interruption. Oracle’s answer to application failover is a new Oracle Net mechanism dubbed Transparent Application Failover. TAF allows the DBA to configure the type and method of failover for each Oracle Net client.
TAF architecture offers the ability to restart transactions at either the transaction (SELECT) or session level.

What are the requirements for Oracle Clusterware?

1. External Shared Disk to store Oracle Cluster ware file (Voting Disk and Oracle Cluster Registry – OCR)
2. Two netwrok cards on each cluster ware node (and three set of IP address) –
Network Card 1 (with IP address set 1) for public network
Network Card 2 (with IP address set 2) for private network (for inter node communication between rac nodes used by clusterware and rac database)
IP address set 3 for Virtual IP (VIP) (used as Virtual IP address for client connection and for connection failover)
3. Storage Option for OCR and Voting Disk – RAW, OCFS2 (Oracle Cluster File System), NFS, …..
Which enable the load balancing of applications in RAC?
Oracle Net Services enable the load balancing of application connections across all of the instances in an Oracle RAC database.

How to find location of OCR file when CRS is down?

If you need to find the location of OCR (Oracle Cluster Registry) but your CRS is down.
When the CRS is down:
Look into “ocr.loc” file, location of this file changes depending on the OS:
On Linux: /etc/oracle/ocr.loc
On Solaris: /var/opt/oracle/ocr.loc
When CRS is UP:
Set ASM environment or CRS environment then run the below command:
ocrcheck

In 2 node RAC, how many NIC’s are r using ?

2 network cards on each clusterware node
Network Card 1 (with IP address set 1) for public network
Network Card 2 (with IP address set 2) for private network (for inter node communication between rac nodes used by clusterware and rac database)

In 2 node RAC, how many IP’s are r using ?

6 – 3 set of IP address
## eth1-Public: 2
## eth0-Private: 2
## VIP: 2

How to find IP’s information in RAC ?

Edit the /etc/hosts file as shown below:
# Do not remove the following line, or various programs
# that requires network functionality will fail.
127.0.0.1 localhost.localdomain localhost
## Public Node names
192.168.10.11 node1-pub.hingu.net node1-pub
192.168.10.22 node2-pub.hingu.net node2-pub
## Private Network (Interconnect)
192.168.0.11 node1-prv node1-prv
192.168.0.22 node2-prv node2-prv
## Private Network (Network Area storage)
192.168.1.11 node1-nas node1-nas
192.168.1.22 node2-nas node2-nas
192.168.1.33 nas-server nas-server
## Virtual IPs
192.168.10.111 node1-vip.hingu.net node1-vip
192.168.10.222 node2-vip.hingu.net node2-vip

What is difference between RAC ip addresses ?

Public IP adress is the normal IP address typically used by DBA and SA to manage storage, system and database. Public IP addresses are reserved for the Internet.
Private IP address is used only for internal clustering processing (Cache Fusion) (aka as interconnect). Private IP addresses are reserved for private networks.
VIP is used by database applications to enable fail over when one cluster node fails. The purpose for having VIP is so client connection can be failover to surviving nodes in case there is failure

Can application developer access the private ip ?

No. private IP address is used only for internal clustering processing (Cache Fusion) (aka as interconnect)
ORACLE – DATAGUARD

What is Dataguard?

Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions. Data Guard maintains these standby databases as copies of the production database. Data Guard can be used with traditional backup, restoration, and cluster techniques to provide a high level of data protection and data availability.

What is DG Broker?

DG Broker “it is the management and monitoring tool”.
Oracle dataguard broker is a distributed management framework that automates and centralizes the creation , maintenance and monitoring of DG configuration.
All management operations can be performed either through OEM, which uses the broker (or) broker specified command-line tool interface “DGMGRL”.

What is the difference between Dataguard and Standby?

Dataguard :
Dataguard is mechanism/tool to maintain standby database.
The dataguard is set up between primary and standby instance .
Data Guard is only available on Enterprise Edition.

Standby Database :
Physical standby database provides a physically identical copy of the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis.
Standby capability is available on Standard Edition.

REFERENCE:
http://neeraj-dba.blogspot.in/2011/06/difference-between-dataguard-and.html
What are the differences between Physical/Logical standby databases? How would you decide which one is best suited for your environment?
Physical standby DB:
As the name, it is physically (datafiles, schema, other physical identity) same copy of the primary database.
It synchronized with the primary database with Apply Redo to the standby DB.
Logical Standby DB:
As the name logical information is the same as the production database, it may be physical structure can be different.
It synchronized with primary database though SQL Apply, Redo received from the primary database into SQL statements and then executing these SQL statements on the standby DB.
We can open “physical stand by DB to “read only” and make it available to the applications users (Only select is allowed during this period). we can not apply redo logs received from primary database at this time.
We do not see such issues with logical standby database. We can open the database in normal mode and make it available to the users. At the same time, we can apply archived logs received from primary database.

For OLTP large transaction database it is better to choose logical standby database.

Explain Active Dataguard?

11g Active Data Guard
Oracle Active Data Guard enables read-only access to a physical standby database for queries, sorting, reporting, web-based access, etc., while continuously applying changes received from the production database.
Oracle Active Data Guard also enables the use of fast incremental backups when offloading backups to a standby database, and can provide additional benefits of high availability and disaster protection against planned or unplanned outages at the production site.

What is a Snapshot Standby Database?

11g Snapshot Standby Database
Oracle 11g introduces the Snapshot Standby database which essentially is an updateable standby database which has been created from a physical standby database.
We can convert a physical standby database to a snapshot standby database, do some kind of testing on a database which is a read write copy of the current primary or production database and then finally revert it to it’s earlier state as a physical standby database.
While the snapshot standby database is open in read-write mode, redo is being received from the primary database, but is not applied.
After converting it back to a physical standby database, it is resynchronized with the primary by applying the accumalated redo data which was earlier shipped from the primary database but not applied.
Using a snapshot standby, we are able to do real time application testing using near real time production data. Very often we are required to do production clones for the purpose of testing. But using snapshot standby databases we can meet the same requirement sparing the effort,time,resources and disk space.

REFERENCE:
http://gavinsoorma.com/2009/07/11g-snapshot-standby-database/

Snapshot Standby Database (UPDATEABLE SNAPSHOT FOR TESTING)
A snapshot standby database is a fully updatable standby database that is created by converting a physical standby database into a snapshot standby database.

Like a physical or logical standby database, a snapshot standby database receives and archives redo data from a primary database. Unlike a physical or logical standby database, a snapshot standby database does not apply the redo data that it receives. The redo data received by a snapshot standby database is not applied until the snapshot standby is converted back into a physical standby database, after first discarding any local updates made to the snapshot standby database.

REFERENCE:
http://docs.oracle.com/cd/B28359_01/server.111/b28294/title.htm

What is the Default mode will the Standby will be, either SYNC or ASYNC?
ASYNC

Dataguard Architechture?

Data Guard Configurations:
A Data Guard configuration consists of one production database and one or more standby databases. The databases in a Data Guard configuration are connected by Oracle Net and may be dispersed geographically. There are no restrictions on where the databases are located, provided they can communicate with each other.

Dataguard Architecture
The Oracle 9i Data Guard architecture incorporates the following items:

• Primary Database – A production database that is used to create standby databases. The archive logs from the primary database are transfered and applied to standby databases. Each standby can only be associated with a single primary database, but a single primary database can be associated with multiple standby databases.
• Standby Database – A replica of the primary database.
• Log Transport Services – Control the automatic transfer of archive redo log files from the primary database to one or more standby destinations.
• Network Configuration – The primary database is connected to one or more standby databases using Oracle Net.
• Log Apply Services – Apply the archived redo logs to the standby database. The Managed Recovery Process (MRP) actually does the work of maintaining and applying the archived redo logs.
• Role Management Services – Control the changing of database roles from primary to standby. The services include switchover, switchback and failover.
• Data Guard Broker – Controls the creation and monitoring of Data Guard. It comes with a GUI and command line interface.

Primary Database:
A Data Guard configuration contains one production database, also referred to as the primary database, that functions in the primary role. This is the database that is accessed by most of your applications.

Standby Database:
A standby database is a transactionally consistent copy of the primary database. Using a backup copy of the primary database, you can create up to nine standby databases and incorporate them in a Data Guard configuration. Once created, Data Guard automatically maintains each standby database by transmitting redo data from the primary database and then applying the redo to the standby database.
The types of standby databases are as follows:

Physical standby database:
Provides a physically identical copy of the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis. The database schema, including indexes, are the same. A physical standby database is kept synchronized with the primary database, through Redo Apply, which recovers the redo data received from the primary database and applies the redo to the physical standby database.

Logical standby database:
Contains the same logical information as the production database, although the physical organization and structure of the data can be different. The logical standby database is kept synchronized with the primary database through SQL Apply, which transforms the data in the redo received from the primary database into SQL statements and then executes the SQL statements on the standby database.

What are the services required on the primary and standby database ?

The services required on the primary database are:
• Log Writer Process (LGWR) – Collects redo information and updates the online redo logs. It can also create local archived redo logs and transmit online redo to standby databases.
• Archiver Process (ARCn) – One or more archiver processes make copies of online redo logs either locally or remotely for standby databases.
• Fetch Archive Log (FAL) Server – Services requests for archive redo logs from FAL clients running on multiple standby databases. Multiple FAL servers can be run on a primary database, one for each FAL request. .
The services required on the standby database are:
• Fetch Archive Log (FAL) Client – Pulls archived redo log files from the primary site. Initiates transfer of archived redo logs when it detects a gap sequence.
• Remote File Server (RFS) – Receives archived and/or standby redo logs from the primary database.
• Archiver (ARCn) Processes – Archives the standby redo logs applied by the managed recovery process (MRP).
• Managed Recovery Process (MRP) – Applies archive redo log information to the standby database.

What is RTS (Redo Transport Services) in Dataguard?

It controls the automated transfer of redo data from the production database to one or more archival destinations. The redo transport services perform the following tasks:
a) Transmit redo data from the primary system to the standby systems in the configuration.
b) Manage the process of resolving any gaps in the archived redo log files due to a network failure.
c) Automatically detect missing or corrupted archived redo log files on a standby system and automatically retrieve replacement archived redo log files from the
primary database or another standby database.

What are the Protection Modes in Dataguard?

Data Guard Protection Modes
This section describes the Data Guard protection modes.
In these descriptions, a synchronized standby database is meant to be one that meets the minimum requirements of the configured data protection mode and that does not have a redo gap. Redo gaps are discussed in Section 6.3.3.

Maximum Availability
This protectionmode provides the highest level of data protection that is possible without compromising the availability of a primary database. Transactions do not commit until all redo data needed to recover those transactions has been written to the online redo log and to at least one synchronized standby database. If the primary database cannot write its redo stream to at least one synchronized standby database, it operates as if it were in maximum performance mode to preserve primary database availability until it is again able to write its redo stream to a synchronized standby database.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.

Maximum Performance
This protectionmode provides the highest level of data protection that is possible without affecting the performance of a primary database. This is accomplished by allowing transactions to commit as soon as all redo data generated by those transactions has been written to the online log. Redo data is also written to one or more standby databases, but this is done asynchronously with respect to transaction commitment, so primary database performance is unaffected by delays in writing redo data to the standby database(s).
This protection mode offers slightly less data protection than maximum availability mode and has minimal impact on primary database performance.
This is the default protection mode.

Maximum Protection
This protection mode ensures that zero data loss occurs if a primary database fails. To provide this level of protection, the redo data needed to recover a transaction must be written to both the online redo log and to at least one synchronized standby database before the transaction commits. To ensure that data loss cannot occur, the primary database will shut down, rather than continue processing transactions, if it cannot write its redo stream to at least one synchronized standby database.
Because this data protection mode prioritizes data protection over primary database availability, Oracle recommends that a minimum of two standby databases be used to protect a primary database that runs in maximum protection mode to prevent a single standby database failure from causing the primary database to shut down.

How to delay the application of logs to a physical standby?
A standby database automatically applies redo logs when they arrive from the primary database. But in some cases, we want to create a time lag between the archiving of a redo log at the primary site, and the application of the log at the standby site.

Modify the LOG_ARCHIVE_DEST_n initialization parameter on the primary database to set a delay for the standby database.

Example: For 60min Delay:
ALTER SYSTEM SET LOG_ARCHIVE_DEST_2=’SERVICE=stdby_srvc DELAY=60′;
The DELAY attribute is expressed in minutes.
The archived redo logs are still automatically copied from the primary site to the standby site, but the logs are not immediately applied to the standby database. The logs are applied when the specified time interval expires.

Steps to create Physical Standby database?

1.Take a full hot backup of Primary database
2.Create standby control file
3.Transfer full backup, init.ora, standby control file to standby node.
4.Modify init.ora file on standby node.
5.Restore database
6.Recover Standby database
(Alternatively, RMAN DUPLICATE DATABASE FOR STANDBY DO RECOVERY can be also used)
7.Setup FAL_CLIENT and FAL_SERVER parameters on both sides
8.Put Standby database in Managed Recover mode

What are the DATAGUARD PARAMETERS in Oracle?

Set Primary Database Initialization Parameters

On the primary database, you define initialization parameters that control redo transport services while the database is in the primary role. There are additional parameters you need to add that control the receipt of the redo data and log apply services when the primary database is transitioned to the standby role.

DB_NAME=chicago
DB_UNIQUE_NAME=chicago
LOG_ARCHIVE_CONFIG=’DG_CONFIG=(chicago,boston)’
CONTROL_FILES=’/arch1/chicago/control1.ctl’, ‘/arch2/chicago/control2.ctl’
LOG_ARCHIVE_DEST_1=
‘LOCATION=/arch1/chicago/
VALID_FOR=(ALL_LOGFILES,ALL_ROLES)
DB_UNIQUE_NAME=chicago’
LOG_ARCHIVE_DEST_2=
‘SERVICE=boston LGWR ASYNC
VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)
DB_UNIQUE_NAME=boston’
LOG_ARCHIVE_DEST_STATE_1=ENABLE
LOG_ARCHIVE_DEST_STATE_2=ENABLE
REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE
LOG_ARCHIVE_FORMAT=%t_%s_%r.arc
LOG_ARCHIVE_MAX_PROCESSES=30
Primary Database: Standby Role Initialization Parameters
FAL_SERVER=boston
FAL_CLIENT=chicago
DB_FILE_NAME_CONVERT=’boston’,’chicago’
LOG_FILE_NAME_CONVERT= ‘/arch1/boston/’,’/arch1/chicago/’,’/arch2/boston/’,’/arch2/chicago/’
STANDBY_FILE_MANAGEMENT=AUTO

Prepare an Initialization Parameter File for the Standby Database

Create a text initialization parameter file (PFILE) from the server parameter file (SPFILE) used by the primary database; a text initialization parameter file can be copied to the standby location and modified. For example:
CREATE PFILE=’/tmp/initboston.ora’ FROM SPFILE;

Modifying Initialization Parameters for a Physical Standby Database.

DB_NAME=chicago
DB_UNIQUE_NAME=boston
LOG_ARCHIVE_CONFIG=’DG_CONFIG=(chicago,boston)’
CONTROL_FILES=’/arch1/boston/control1.ctl’, ‘/arch2/boston/control2.ctl’
DB_FILE_NAME_CONVERT=’chicago’,’boston’
LOG_FILE_NAME_CONVERT= ‘/arch1/chicago/’,’/arch1/boston/’,’/arch2/chicago/’,’/arch2/boston/’
LOG_ARCHIVE_FORMAT=log%t_%s_%r.arc
LOG_ARCHIVE_DEST_1= ‘LOCATION=/arch1/boston/
VALID_FOR=(ALL_LOGFILES,ALL_ROLES)
DB_UNIQUE_NAME=boston’
LOG_ARCHIVE_DEST_2= ‘SERVICE=chicago LGWR ASYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=chicago’
LOG_ARCHIVE_DEST_STATE_1=ENABLE
LOG_ARCHIVE_DEST_STATE_2=ENABLE
REMOTE_LOGIN_PASSWORDFILE=EXCLUSIVE
STANDBY_FILE_MANAGEMENT=AUTO
FAL_SERVER=chicago
FAL_CLIENT=boston

Oracle Performance Tuning
Application user is complaining the database is slow.How would you find the performance issue of SQL queries?

High performance is common expectation for end user, in fact the database is never slow or fast in most of the case session connected to the database slow down when they receives unexpected hit. Thus to solve this issue you need to find those unexpected hit. To know exactly what the session is doing join your query v$session with v$session_wait.
SELECT NVL(s.username,’(oracle)’) as username,s.sid,s.serial#,sw.event,sw.wait_time, sw.seconds_in_wait, sw.state FROM v$session_wait sw,v$session s
WHERE s.sid=sw.sid and s.username= ‘&username’ORDER BY sw.seconds_in_wait DESC;

1.Check the events that are waiting for something.
2.Try to find out the objects locks for that particular session.
3.Locking is not only the cause to effects the performance. Disk I/O contention is another case. When a session retrieves data from the database datafiles on disk to the buffer cache, it has to wait until the disk sends the data. The wait event shows up for the session as “db file sequential read” (for index scan) or “db file scattered read” (for full table scan).When you see the event, you know that the session is waiting for I/O from the disk to complete. To improve session performance, you have to reduce that waiting period. The exact step depends on specific situation, but the first technique “reducing the number of blocks retrieved by a SQL statement” almost always works.Reduce the number of blocks retrieved by the SQL statement. Examine the SQL statement to see if it is doing a full-table scan when it should be using an index, if it is using a wrong index, or if it can be rewritten to reduce the amount of data it retrieves.
4.Run SQL Tuning Advisor (@$ORACLE_HOME/rdbms/admin/sqltrpt.sql) by providing SQL_ID as the input for generating the findings and recommendations.
SQL Tuning Advisor seems to be doing logical optimization mainly by checking your SQL structure and statistics.
SQL Tuning Advisor suggests indexes that might be very useful.
SQL Tuning Advisor suggests query rewrites.
SQL Tuning Advisor suggests SQL profile.

More:
1.Run TOP command in Linux to check CPU usage.
2.Run VMSTAT, SAR, PRSTAT command to get more information on CPU, memory usage and possible blocking.
3.Enable the trace file before running your queries,then check the trace file using tkprof create output file.
According to explain plan check the elapsed time for each query,then tune them respectively.

What is the use of iostat/vmstat/netstat command in Linux?

Iostat – reports on terminal, disk and tape I/O activity.
Vmstat – reports on virtual memory statistics for processes, disk, tape and CPU activity.
Netstat – reports on the contents of network data structures.

If you are getting high “Busy Buffer waits”, how can you find the reason behind it?
Buffer busy wait means that the queries are waiting for the blocks to be read into the db cache. There could be the reason when the block may be busy in the cache and session is waiting for it. It could be undo/data block or segment header wait.
Run the below two query to find out the P1, P2 and P3 of a session causing buffer busy wait
then after another query by putting the above P1, P2 and P3 values.
SQL> Select p1 “File #”,p2 “Block #”,p3 “Reason Code” from v$session_wait Where event = ‘buffer busy waits’;
SQL> Select owner, segment_name, segment_type from dba_extents
Where file_id = &P1 and &P2 between block_id and block_id + blocks -1;

What to Look for in AWR Report and STATSPACK Report?

Many DBAs already know how to use STATSPACK but are not always sure what to check regularly.
Remember to separate OLTP and Batch activity when you run STATSPACK, since they usually
generate different types of waits. The SQL script “spauto.sql” can be used to run STATSPACK
every hour on the hour. See the script in $ORACLE_HOME/rdbms/admin/spauto.sql for more
information (note that JOB_QUEUE_PROCESSES must be set > 0). Since every system is different,this is only a general list of things you should regularly check in your STATSPACK output:

¦ Top 5 wait events (timed events)
¦ Load profile
¦ Instance efficiency hit ratios
¦ Wait events
¦ Latch waits
¦ Top SQL
¦ Instance activity
¦ File I/O and segment statistics
¦ Memory allocation
¦ Buffer waits

What is the difference between DB file sequential read and DB File Scattered Read?
DB file sequential read is associated with index read where as DB File Scattered Read has to do with full table scan.
The DB file sequential read, reads block into contiguous memory and DB File scattered read gets from multiple block and scattered them into buffer cache.

Which factors are to be considered for creating index on Table? How to select column for index?

Creation of index on table depends on size of table, volume of data. If size of table is large and we need only few data for selecting or in report then we need to create index. There are some basic reason of selecting column for indexing like cardinality and frequent usage in where condition of select query. Business rule is also forcing to create index like primary key, because configuring primary key or unique key automatically create unique index.
It is important to note that creation of so many indexes would affect the performance of DML on table because in single transaction should need to perform on various index segments and table simultaneously.

Is creating index online possible?

YES. You can create and rebuild indexes online. This enables you to update base tables at the same time you are building or rebuilding indexes on that table. You can perform DML operations while the index building is taking place, but DDL operations are not allowed. Parallel execution is not supported when creating or rebuilding an index online.
CREATE INDEX emp_name ON emp (mgr, emp1, emp2, emp3) ONLINE;

How to recover password in oracle 10g?

You can query with the table user_history$. The password history is store in this table.

How can you track the password change for a user in oracle?

Oracle only tracks the date that the password will expire based on when it was latest changed. Thus listing the view DBA_USERS.EXPIRY_DATE and subtracting PASSWORD_LIFE_TIME you can determine when password was last changed. You can also check the last password change time directly from the PTIME column in USER$ table (on which DBA_USERS view is based). But If you have PASSWORD_REUSE_TIME and/or PASSWORD_REUSE_MAX set in a profile assigned to a user account then you can reference dictionary table USER_HISTORY$ for when the password was changed for this account.
SELECT user$.NAME, user$.PASSWORD, user$.ptime, user_history$.password_date
FROM SYS.user_history$, SYS.user$
WHERE user_history$.user# = user$.user#;

What is Secure External password Store (SEPS)?

Through the use of SEPS you can store password credentials for connecting to database by using a client side oracle wallet, this wallet stores signing credentials. This feature introduced since oracle 10g. Thus the application code, scheduled job, scripts no longer needed embedded username and passwords. This reduces risk because the passwords are no longer exposed and password management policies are more easily enforced without changing application code whenever username and password change.

Why we need CASCADE option with DROP USER command whenever dropping a user and why “DROP USER” commands fails when we don’t use it?

If a user having any object then ‘YES’ in that case you are not able to drop that user without using CASCADE option. The DROP USER with CASCADE option command drops user along with its all associated objects. Remember it is a DDL command after the execution of this command rollback cannot be performed.

What is the difference between Redo,Rollback and Undo?

I find there is always some confusion when talking about Redo, Rollback and Undo. They all sound like pretty much the same thing or at least pretty close.
Redo: Every Oracle database has a set of (two or more) redo log files. The redo log records all changes made to data, including both uncommitted and committed changes. In addition to the online redo logs Oracle also stores archive redo logs. All redo logs are used in recovery situations.
Rollback: More specifically rollback segments. Rollback segments store the data as it was before changes were made. This is in contrast to the redo log which is a record of the insert/update/deletes.
Undo: Rollback segments. They both are really one in the same. Undo data is stored in the undo tablespace. Undo is helpful in building a read consistent view of data.

You have more than 3 instances running on the Linux server? How can you determine which shared memory and semaphores are associated with which instance?
Oradebug is undocumented oracle supplied utility by oracle. The oradebug help command list the command available with oracle.
SQL>oradebug setmypid
SQL>oradebug ipc
SQL>oradebug tracfile_name

Why drop table is not going into Recycle bin?

If you are using SYS user to drop any table then user’s object will not go to the recyclebin as there is no recyclebin for SYSTEM tablespace, even we have already SET recycle bin parameter TRUE.
Select * from v$parameter where name = ‘recyclebin’;
Show parameter recyclebin;

Temp Tablespace is 100% FULL and there is no space available to add datafiles to increase temp tablespace. What can you do in that case to free up TEMP tablespace?
Try to close some of the idle sessions connected to the database will help you to free some TEMP space. Otherwise you can also use ‘Alter Tablespace PCTINCREASE 1’ followed by ‘Alter Tablespace PCTINCREASE 0’

What is Row Chaning and Row Migration?

Row Migration:
A row migrates when an update to that row would cause it to not fit on the block anymore (with all of the other data that exists there currently). A migration means that the entire row will move and we just leave behind the «forwarding address». So, the original block just has the rowid of the new block and the entire row is moved.

Row Chaining:
A row is too large to fit into a single database block. For example, if you use a 4KB blocksize for your database, and you need to insert a row of 8KB into it, Oracle will use 3 blocks and store the row in pieces.
Some conditions that will cause row chaining are: Tables whose rowsize exceeds the blocksize. Tables with LONG and LONG RAW columns are prone to having chained rows. Tables with more then 255 columns will have chained rows as Oracle break wide tables up into pieces.
So, instead of just having a forwarding address on one block and the data on another we have data on two or more blocks.

How to find out background processes ?

SQL> select SID,PROGRAM from v$session where TYPE=’BACKGROUND’;
SQL> select name,description from V$bgprocess;

How to findout background processes from OS:

$ ps -ef|grep ora_|grep SID
To Find and Delete bigger size and older files in Linux
–To find out files size more than 5MB
find . -size +5000 -exec ls -ltr {} \;
– To **Remove** files size more than 5MB
find . -size +5000k -exec rm -rf {} \;
–To find out files older than 30days
find . -mtime +30 -exec ls -ltr {} \;
–To find **Remove** files older than 30days
find . -mtime +30 -exec rm -rf {} \;