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Oracle Data Guard Failover Options

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Oracle Data guard Failover Options

Each of these options will incur different downtimes, configuration costs and expenses.  Since Oracle introduced recovery products 12 years ago, their technologies have evolved significantly:

Traditional recovery (1990-1995) – This recovery method requires restoration of failed database files and a roll-forward using Oracle’s Enterprise Backup Utility (EBU) or the Oracle8 Recovery Manager (RMAN) utility. This type of recovery could take several hours.

Manual Standby databases (1993-present) - Oracle7 introduced mechanisms that allow a Oracle instance to be constantly in recovery mode and to be refreshed from Oracle’s archived redo logs. In case of failure, the last redo log could be added to the standby database, and the database could be started in just a few minutes.

Standby database is not an Oracle product, but a procedure that was used prior to Oracle Data Guard to create a standby database.  Oracle7 introduced mechanisms that allow a Oracle instance to be constantly in recovery mode and to be refreshed from Oracle’s archived redo logs. In case of failure, the last redo log could be added to the standby database, and the database could be started in just a few minutes.

Oracle Parallel Server (1996-2001) - The OPS architecture allowed for several Oracle instances to share a common set of database files. In case of instance failure, the surviving instances could take over processing. There was a significant performance issue with OPS because shared RAM blocks had to be “pinged” between instances, imposing an additional processing burden on the cluster.

Oracle Failover Options

Replication failover
– Multi-master replication can allow the Oracle instance to be open and accepting transactions, with DML cross-feeding each instance.  This requires Oracle Enterprise Edition and is only for databases with low DML rates

Real Application Clusters (2001–present) - The RAC architecture allows many instances to share a single database, but it avoids the overhead of RAM block pinging. RAC has also been enhanced to work with Oracle’s Transparent Application Failover (TAF) to automatically restart any connections when an instance fails.

Oracle Streams - This is a high-speed replication solution that takes SQL directly from the log_buffer  RAM area and replicates transactions to a remote database.  During a server crash, transactions can be quickly redirected to the replicated system.

Oracle Data Guard - This is a free option with Oracle Enterprise Edition, and it provides an automated standby database.  Upon server failure, a series of database procedures synchronizes the Oracle instance and opens it to accept connections. Oracle Data Guard is free with Enterprise Edition.

Data Guard Failover and Oracle licensing

Cost-conscious Oracle shops want to know how they can get Oracle failover as cheaply as possible.  In addition to Data Guard, Oracle has a wealth of failover options, each with its own licensing costs.  Oracle has two licensing tiers, Enterprise Edition (EE) and the far cheaper Standard Edition (SE).

Data Guard Switchover or failover operation

The Oracle instance will be activated to serve as the primary database at some point in its life cycle. There are normally two situations when this operation will be performed: a planned outage for maintenance of the primary database or disaster recovery. A switchover operation occurs when a Oracle instance is transitioned into the primary database role and the primary database into the Oracle instance role. In the switchover operation, no data is lost.

The switchover operation is performed for maintenance of the primary database. In case of an unplanned outage on the primary site, the Oracle instance will be activated as the primary database. This is called failover. There are two types of failover operations: Graceful or “no-data-loss” failover and Forced or “minimal-data-loss” failover.  Once the Oracle instance is transitioned into primary database status in either switchover or failover, the life of the database as the standby ends and its service as the primary database begins.



The above text is an excerpt from:

Oracle Data Guard
ISBN 0-9745993-8-7 Guard.htm

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