Real Application Clusters (RAC)
An RAC database is a clustered database. In RAC environment, multiple Oracle instances (which are connected via interconnect) operate on same database files on disk. Different servers must access set of shared disks and should communicate through high-speed interconnection. Changed database blocks and log information on one instance can be moved to another instance through memory interconnection without writing on disk. This is known as cache fusion (see later). The main advantage of RAC is scalability and high availability. Several servers with Oracle instances can be added to system. If one of them fails, others continue to work without affecting operations. For this reason RAC is also known as High Availability. When work load grows, you can simply add another server to the grid (RAC is a type of grid computing after all).
The data files are stored in several disk drives which are connected by cluster aware storage. By adding multiple instances, we can add and remove and single Oracle instance without bringing the database down. So, database always remains available. That is the essence of RAC - high availability.
An RAC database is managed by Oracle Clusterware. RAC operates in shared everything architecture mode.
Any storage (eg. SAN, SCSI etc.) can be used with RAC. However, good I/O speed is required for scalability of data volume.
RAC supports up to 100 clusters which may be different at hardware level but must run same operating system.
Oracle recommends using ASM (Automatic Storage Management) for ease of dealing with clustered storage.
An Oracle RAC database requires three components - cluster nodes (the servers or computers running Oracle instances), shared storage (disk drives) and Oracle Clusterware (software application).
Installing RAC
The first step of working with RAC is to install "Oracle Clusterware" which is installed via Universal Installer.
Then you have to configure the clusterware.
Then install ASM (Automatic Storage Management).
Now install Oracle 11g database.
Then perform post installation tasks. This ensures that clusterware and database are installed properly and they are aware of each other.
It is possible to convert your normal single instance Oracle database to an RAC database. You can achieve this via Enterprise Manager or rconfig utility.
Administering Clusterware
Oracle Clusterware includes two important components: the voting disk and the OCR. The voting disk is a file that manages information about node membership, and the OCR is a file that manages cluster and Oracle RAC database configuration information.
RAC can be administered via Oracle Enterprise manager. On EM's web console mode, click on Availability tab to see details of Clusterware. You can click on Topology tab to see a visual representation of your nodes. The Interconnect tab shows you info on interfaces. You can also add new instance to clusterware via
EM (under Server tab).
Oracle Clusterware posts alert messages in alert log – which is under $CRS_home. RAC data dictionary views are created by catclust.sql.
Cache Fusion
Oracle RAC uses Cache Fusion to synchronize the data stored in the buffer cache of each database instance – i.e. to keep track of which nodes are writing to which blocks and to ensure that two nodes do not update duplicate copies of the same block. Since all computers/instances in an RAC access the same database, the overall system must guarantee the coordination of data changes on different computers such that whenever a computer queries data it receives the current version – even if another computer recently modified that data. Oracle RAC refers to this functionality as Cache Fusion. It involves the ability of RAC to fuse in-memory data cached physically separately on each computer into a single global cache. This saves a lot of resource time.
The data files are stored in several disk drives which are connected by cluster aware storage. By adding multiple instances, we can add and remove and single Oracle instance without bringing the database down. So, database always remains available. That is the essence of RAC - high availability.
An RAC database is managed by Oracle Clusterware. RAC operates in shared everything architecture mode.
Any storage (eg. SAN, SCSI etc.) can be used with RAC. However, good I/O speed is required for scalability of data volume.
RAC supports up to 100 clusters which may be different at hardware level but must run same operating system.
Oracle recommends using ASM (Automatic Storage Management) for ease of dealing with clustered storage.
An Oracle RAC database requires three components - cluster nodes (the servers or computers running Oracle instances), shared storage (disk drives) and Oracle Clusterware (software application).
Installing RAC
The first step of working with RAC is to install "Oracle Clusterware" which is installed via Universal Installer.
Then you have to configure the clusterware.
Then install ASM (Automatic Storage Management).
Now install Oracle 11g database.
Then perform post installation tasks. This ensures that clusterware and database are installed properly and they are aware of each other.
It is possible to convert your normal single instance Oracle database to an RAC database. You can achieve this via Enterprise Manager or rconfig utility.
Administering Clusterware
Oracle Clusterware includes two important components: the voting disk and the OCR. The voting disk is a file that manages information about node membership, and the OCR is a file that manages cluster and Oracle RAC database configuration information.
RAC can be administered via Oracle Enterprise manager. On EM's web console mode, click on Availability tab to see details of Clusterware. You can click on Topology tab to see a visual representation of your nodes. The Interconnect tab shows you info on interfaces. You can also add new instance to clusterware via
EM (under Server tab).
Oracle Clusterware posts alert messages in alert log – which is under $CRS_home. RAC data dictionary views are created by catclust.sql.
Cache Fusion
Oracle RAC uses Cache Fusion to synchronize the data stored in the buffer cache of each database instance – i.e. to keep track of which nodes are writing to which blocks and to ensure that two nodes do not update duplicate copies of the same block. Since all computers/instances in an RAC access the same database, the overall system must guarantee the coordination of data changes on different computers such that whenever a computer queries data it receives the current version – even if another computer recently modified that data. Oracle RAC refers to this functionality as Cache Fusion. It involves the ability of RAC to fuse in-memory data cached physically separately on each computer into a single global cache. This saves a lot of resource time.
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