The
SmartSync feature of 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. 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.
You must configure volumes correctly to use SmartSync. For VxVM, there
are two types of volumes used by the database, as follows:
Redo log volumes contain redo logs
of the database.
Data volumes are all other volumes used by the database
(control files and tablespace files).
SmartSync works with these two types of volumes differently,
and they must be configured correctly to take full advantage of
the extended interfaces. The only difference between the two types
of volumes is that redo log volumes have dirty region logs, while
data volumes do not.
To enable the use of SmartSync with database volumes in shared
disk groups, set the value of the volcvm_smartsync tunable to 1
as described in “Tuning VxVM”.
See “volcvm_smartsync” for more
information about this tunable.
Data
Volume Configuration |
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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
which improves normal database write access.
Redo
Log Volume Configuration |
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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.
For details of how to configure sequential DRL, see “Adding
DRL Logging to a Mirrored Volume”.
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