The VxSmartSync™ Recovery Accelerator is available
for some systems. VxSmartSync for Mirrored Oracle© Databases
is a collection of features that speed up the resynchronization
process (known as resilvering) for volumes
used in with the Oracle Universal Database™. These features use
an extended interface between Volume Manager volumes and the database
software so they can avoid unnecessary work during mirror resynchronization.
These extensions can result in an order of magnitude improvement
in volume recovery times.
Oracle automatically takes advantage of SmartSync when it
is available.
The system administrator must configure the volumes correctly
to use VxSmartSync. For Volume Manager, there are two types of volumes
used by the database:
redo log volumes contain redo logs of the database.
data volumes are all other volumes used by the database (control files
and tablespace files).
VxSmartSync 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 should have dirty region logs,
while data volumes should not.
Data Volume Configuration |
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The improvement in recovery time for data volumes is achieved
by letting the database software decide which portions of the volume require
recovery. The database keeps logs of changes to the data in the database
and can determine which portions of the volume require recovery.
By reducing the amount of space that requires recovery and allowing
the database to control the recovery process, the overall recovery
time is reduced.
Also, 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 Volume Manager to do logging. Data volumes should therefore
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. No logs of the changes to the redo logs are kept
by the database, so the database itself 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, Volume Manager can reduce
the number of dirty regions by modifying the behavior of its Dirty
Region Logging feature to take advantage of sequential access patterns.
This 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 Volume Manager when a volume is to be used as a redo log.
This allows Volume Manager 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 dirty region logs.
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