VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

SmartSync Recovery Accelerator
50 VERITAS Volume Manager Administrator’s Guide
You mustconfigurevolumes correctly to useSmartSync. For VxVM,therearetwo 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” on
page 291. See “volcvm_smartsync” on page 296 for more information about this tunable.
Data Volume Configuration
The recoverytakes placewhenthe database softwareis started,notat systemstartup. This
reduces the overall impact of recovery when the system reboots. Because the recovery is
controlled by the database, therecovery 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
A redo log isa log of changesto the database data. Because the databasedoes 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
sequentialaccess patterns.Sequential DRL decreasesthe amount ofdataneedingrecovery
and reduces recovery time impact on the system.
The enhanced interfaces for redo logs allow the database software to inform VxVM when
a volumeis tobe usedas a redo log.This allowsVxVM 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.