The SmartSync feature of Veritas Volume Manager increases the availability of mirrored volumes by only resynchronizing changed data. (The process of resynchronizing mirrored databases is also sometimes referred to as resilvering.) SmartSync reduces the time required to restore consistency, freeing more I/O bandwidth for business-critical applications. If supported by the database vendor, the SmartSync feature uses an extended interface between VxVM volumes and the database software to avoid unnecessary work during mirror resynchronization. For example, Oracle® automatically takes advantage of SmartSync to perform database resynchronization when it is available.
Note The SmartSync feature of Veritas Volume Manager is only applicable to databases that are configured on raw volumes. You cannot use it with volumes that contain file systems. Use an alternative solution such as the Oracle Resilvering feature of Veritas File System (VxFS).
You must configure volumes correctly to use SmartSync. For VxVM, there are two types of volumes used by the database, as follows:
SmartSync works with these two types of volumes differently, so they must be configured as described in the following sections.
To enable the use of SmartSync with database volumes in shared disk groups, set the value of the
volcvm_smartsync tunable to
See "Tunable parameters" on page 476.
The recovery takes place when the database software is started, not at system startup. This reduces the overall impact of recovery when the system reboots. Because the recovery is controlled by the database, the recovery time for the volume is the resilvering time for the database (that is, the time required to replay the redo logs).
Because the database keeps its own logs, it is not necessary for VxVM to do logging. Data volumes should be configured as mirrored volumes without dirty region logs. In addition to improving recovery time, this avoids any run-time I/O overhead due to DRL, and improves normal database write access.
A redo log is a log of changes to the database data. Because the database does not maintain changes to the redo logs, it cannot provide information about which sections require resilvering. Redo logs are also written sequentially, and since traditional dirty region logs are most useful with randomly-written data, they are of minimal use for reducing recovery time for redo logs. However, VxVM can reduce the number of dirty regions by modifying the behavior of its dirty region logging feature to take advantage of sequential access patterns. Sequential DRL decreases the amount of data needing recovery and reduces recovery time impact on the system.
The enhanced interfaces for redo logs allow the database software to inform VxVM when a volume is to be used as a redo log. This allows VxVM to modify the DRL behavior of the volume to take advantage of the access patterns. Since the improved recovery time depends on dirty region logs, redo log volumes should be configured as mirrored volumes with sequential DRL.
See "Sequential DRL" on page 62.