VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
VxSmartSync Recovery Accelerator
Chapter 3 127
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
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.