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Donald K. Burleson

Oracle9i RAC Tips

Oracle Database Service in HA Cluster

The Oracle database is a widely used database system. Large numbers of critical applications and business operations depend on the availability of the database. Most of the cluster products provide agents to support database failover processes. The implementation of an Oracle database service with failover in a HA cluster has the following general features:

  • A single instance of Oracle runs on one of the nodes in the cluster. The Oracle instance and the listener are dependent on other resources such as file systems, mount points, and IP addresses.
  • It has exclusive access to the set of database disk groups on a storage array that is shared among the nodes.
  • Optionally, an Active/Active architecture of Oracle databases can be established. One node (Node1) acts as the primary node to an Oracle instance (db1) and another node acts as a secondary node (Node2) for failover purposes. At the same time, the secondary node (node2) acts as primary for another instance (db2) and the primary node acts as the backup/secondary node.
  • When the primary node suffers a failure, the Oracle instance is restarted on the surviving or backup node in the cluster.
  • The failover process involves configuring the IP address, importing the disk group, starting the volumes, and mounting the file systems containing the Oracle data files to the backup node.
  • Restarting the database automatically performs crash recovery, returning the database to a transactional consistent state.

There are a few issues regarding Oracle database failover that one should be aware:

  • On restarting the database, a fresh database cache (SGA) is established and the previous instance’s SGA contents are lost.
  • Once the new instance is created and made available on the backup node, all client connections seeking the database service attempt to connect at the same time. This could result in a lengthy waiting period.
  • The impact of the outage may be felt for an extended duration during the failover process. When there is a failure at the primary node, all the relevant resources such as mount points, disk groups, listeners, and database instances have to be logically off-lined or shutdown. This process may take some time depending on the failure situation.

When the Oracle database cluster is implemented in a parallel, scalable cluster such as Oracle RAC, it provides a transparent failover for the clients. Other high availability features and advantages include:

  • Multiple instances accessing a single database exist at the same time. (Data files are common to the multiple instances.)
  • Multiple nodes have read-write access to the shared storage at the same time. Data blocks are read and updated by multiple nodes.
  • Should a failure occur in a node and the Oracle instance is not usable or has crashed, the surviving node performs recovery for the crashed instance. There is no need to restart the instance on the surviving node, since a parallel instance is already running there.
  • All the client connections continue to access the database through the surviving node/instance. With the TAF (Transparent Application Failover) facility, clients will be able to move over to the surviving instance almost instantaneously.
  • IP addresses, volumes, and file systems do not have to be moved to the surviving node.

Some of the well-known failover cluster products supported for Oracle databases are:

  • Veritas Cluster Server
  • HP Service Guard
  • Microsoft Cluster Service with Oracle Failsafe
  • RedHat Linux Advanced Server 2.1
  • Sun Cluster Oracle Agent
  • Compaq, now HP, Segregated Cluster
  • HACMP

For more information, see the book Oracle 11g Grid and Real Application Clusters  - 30% off if you buy it directly from Rampant TechPress .  Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.

For more details and scripts, see my new book " Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot.
 
 


 

 

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