RAC stands for Real Application Clusters. It allows multiple nodes in a clustered system to mount and open a single database that resides on shared disk storage. Should a single system fail (node), the database service will still be available on the remaining nodes.
In RAC database, comprises of multiple instances, however there will be only one database.
A non-RAC database is only available on a single system. If that system fails, the database service will be down (single point of failure).
Oracle Database 10g Real Application Clusters (RAC) enables the clustering of the Oracle Database. A RAC database comprises of multiple instances residing on different computers to access a common database residing on shared storage.
Why Real Application Cluster ?
The basic principle behind the Real Application Cluster is greater throughput and scalability due to the combined power of multiple instances running on multiple servers
Real Application Cluster provides high availability and scalability for all application types. The RAC infrastructure is also a key component for implementing the Oracle enterprise grid computing architecture. Multiple instances access a single physical database prevents the server from being a single point of failure. Combining smaller servers into a cluster can be achieved to create scalable environment that support mission critical business applications.
Real application Cluster uses Oracle Cluster ware for the infrastructure to bind multiple servers so that they operate as a single system.
Oracle Clusterware is a portable cluster management solution that is integrated with the Oracle database. The Oracle Clusterware is also a required component for using Real application Cluster.
In Oracle Database 10g, Oracle Provides you with an integrated software solution that addresses cluster management, event management, application management, connection management, storage management, load balancing and availability. These capabilities are addressed while hiding the complexity through simple-to-use management tools and automation.
Oracle Real Application Clusters 10g provides an integrated cluster ware layer that delivers a complete environment for applications.
Oracle Real Application Cluster (RAC) uses Oracle Clusterware for the infrastructure to bind multiple servers so that they operate as a single system
Main Components of Oracle Real Application Cluster 10g
It comprises of two main components
- Oracle Clusterware
- Oracle Software
In RAC 10g Clusterware is called CRS layer which resides below Oracle software Layer. Second layer is the Oracle software itself.
Oracle Real Application Cluster (RAC) is the Oracle Database option that provides a single system image for multiple servers to access one Oracle database. In RAC, each Oracle instance usually runs on a separate server.
Oracle Clusterware is software, enables the servers to be bound together to operate as if they were one server. The Oracle Clusterware comprises of two clusterware components: a voting disk to record node membership information and the Oracle Cluster Registry (OCR) to record cluster configuration information. In Oracle Clusterware each node is connected to a private network by way of a private interconnect.
The Oracle Clusterware comprises several background processes that facilitate cluster operations such as Cluster Synchronization Service (CSS) and Event Management (EVM).
What are the Real Application Cluster Main Processes ?
The main processes involved in Oracle RAC are primarily used to maintain database coherency among each instance. They manage what is called the global resources.
- LMON: Global Enqueue Service Monitor
- LMD0: Global Enqueue Service Daemon
- LMSx: Global Cache Service Processes. Where x can range from 0 to j
- LCK0: Lock Process
- DIAG : Diagnostibility Process
There are several tools that are used to manage the various resources available on the cluster at a global level. Some of the tools used are Server Control (SRVCTL), DBCA and Enterprise Manager.
Oracle Clusterware is a portable cluster management solution that is integrated with the Oracle database. The Oracle Clusterware enables you to create a clustered pool of storage to be used by any combination of single-instance and RAC databases.
Oracle Clusterware is the only clusterware that you need for most platforms on which RAC operates. You can also use clusterware from other vendors if the clusterware is certified for RAC.
The combined processing power of the multiple servers can provide greater throughput and scalability than is available from a single server. RAC is the Oracle Database option that provides a single system image for multiple servers to access one Oracle database. In RAC, each Oracle instance runs on a separate server.
RAC is a unique technology that provides high availability and scalability for all application types. The RAC infrastructure is also a key component for implementing the Oracle enterprise grid computing architecture. Having multiple instances access a single database prevents the server from being a single point of failure. RAC enables you to combine smaller commodity servers into a cluster to create scalable environments that support mission critical business applications
What are the Storage Principles for RAC Software and CRS ?
The Oracle Software 10g Real Application Clusters installation is a two-phase installation in the first Phase, You install CRS. In the second phase, you install the Oracle Database software with RAC components and create a cluster database.
The oracle home that you use for the CRS software must be different from the one that is used for the RAC Software. CRS and RAC software is installed on cluster shared storage.
Note:- Cluster Software and Oracle software is usually installed on a regular file system that is local to each node. This permits online patch upgrades without shutting down the database and also eliminates the software as a single point of failure.
Do you need special hardware to run RAC ?
RAC requires the following hardware components:
- A dedicated network interconnect - might be as simple as a fast network connection between nodes; and a shared disk subsystem.
RAC and Shared Storage Technologies
- Supported shared storage for Oracle grids:
- Network attached Storage
- Storage Area Network
- Supported file systems for Oracle grids:-
- Raw Volumes
- Cluster file system
- ASM
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