Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

58 Understanding Veritas Volume Manager
Dirty region logging
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:
■ Data volumes are all other volumes used by the database (control files and tablespace
files).
■ Redo log volumes contain redo logs of the database.
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 1. For a description of volcvm_smartsync,
see “Tunable parameters” on page 463.
Data volume configuration
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.
Redo log volume configuration
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