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What Happens When Fast-Start Failover and the Observer Are Running?

原创 Linux操作系统 作者:liyijie78 时间:2011-02-10 13:55:08 0 删除 编辑

Once you enable fast-start failover and start the observer, the observer continuously monitors the environment to ensure the primary database is available. This section lists the steps the observer takes to determine if fast-start failover is needed and then performs one, if necessary.

Step 1   Monitor the environment to ensure the primary database is available.

The following list describes some of the conditions with the primary database, system, or site that could cause the observer to attempt a fast-start failover:

  • Broken network connection between the observer and the primary database

    If the connection is lost between the observer and the primary database, or there are network failures that cause the primary database to be isolated, the observer attempts a fast-start failover.

  • Instance failures

    If a single-instance primary database (either RAC or nonRAC), or if all instances of a RAC primary database fail, the observer attempts a fast-start failover.

  • Shutdown abort

    If a single-instance primary database (either RAC or nonRAC), or if all instances of a RAC primary database are shut down with the ABORT option, the observer attempts a fast-start failover. Fast-start failover will not be attempted for the other types of database shutdown (NORMAL, IMMEDIATE, TRANSACTIONAL).

  • Offline datafiles

    If the observer determines that one or more datafiles in the primary database have been taken offline by the database because of I/O errors, the observer attempts a fast-start failover.

Except for the last condition (Offline datafiles), the observer attempts to reconnect to the primary database within the time specified by the FastStartFailoverThreshold configuration property before attempting a fast-start failover. When the primary database datafiles are offline, the observer initiates a fast-start failover immediately, without waiting for the amount of time specified by the FastStartFailoverThreshold property to expire.

Step 2   Reconnect within the time specified by FastStartFailoverThreshold.

If the observer detects a problem, the observer attempts to reconnect to the primary database within the time specified by the FastStartFailoverThreshold property. The FastStartFailoverThreshold time interval starts when the observer first detects there might be a failure with the primary database.

If the primary database is a Real Application Clusters (RAC) database, the observer will attempt to connect to the remaining primary instances. Fast-start failover will not occur unless all instances comprising the RAC primary database are perceived to have failed.


Step 3   Verify the target standby database is ready for failover.

If the primary database is still unavailable when the FastStartFailoverThreshold expires, the observer verifies the target standby database is ready to fail over to the primary database role.

Fast-start failover cannot occur if:

  • Fast-start failover is no longer enabled

  • The observer cannot connect to the target standby database

  • The observer and the target standby database are inconsistent with regard to the current state of the broker configuration

  • The observer was stopped, and when it restarted, only the standby database was available

  • The target standby database was not synchronized with the primary database at the time the primary database failed

  • The target standby database, if it is a logical standby database, is still loading its copy of the former primary database's dictionary (in this case the FS_FAILOVER_STATUS column in the V$DATABASE view contains LOADING DICTIONARY instead of READY)

  • The network connection exists between the target standby database and the primary database

  • The FS_FAILOVER_STATUS column in the V$DATABASE view for the target standby database displays a reason why fast-start failover cannot occur

  • A manual failover is already in progress.

  • The primary database was shut down intentionally, in a controlled fashion

Step 4   Initiate a fast-start failover.

If the target standby database is ready for failover, the observer immediately invokes a fast-start failover. The observer will fail over the target standby database to the primary database role. If failover is not possible for some reason, the observer will try again to connect to the primary database indefinitely.

Step 5   Reinstate the former primary database as a new standby database.

After the fast-start failover completes successfully, the observer will attempt to reinstate the former primary database as a new standby database when a connection to the former primary database is reestablished.

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