Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

Resynchronization can impact system performance. The recovery process reduces
some of this impact by spreading the recoveries to avoid stressing a specific disk
or controller.
For large volumes or for a large number of volumes, the resynchronization process
can take time. These effects can be minimized by using dirty region logging (DRL)
and FastResync (fast mirror resynchronization) for mirrored volumes, or by using
RAID-5 logs for RAID-5 volumes.
See “Dirty region logging” on page 60.
See “FastResync” on page 65.
For mirrored volumes used by Oracle, you can use the SmartSync feature, which
further improves performance.
See “SmartSync recovery accelerator” on page 61.
Dirty region logging
Note: You need a full license to use this feature.
Dirty region logging (DRL), if enabled, speeds recovery of mirrored volumes after
a system crash. DRL tracks the regions that have changed due to I/O writes to a
mirrored volume. DRL uses this information to recover only those portions of the
volume.
If DRL is not used and a system failure occurs, all mirrors of the volumes must be
restored to a consistent state. Restoration is done by copying the full contents of
the volume between its mirrors. This process can be lengthy and I/O intensive.
Note: DRL adds a small I/O overhead for most write access patterns. This overhead
is reduced by using SmartSync.
If a version 20 DCO volume is associated with a volume, a portion of the DCO
volume can be used to store the DRL log. There is no need to create a separate
DRL log for a volume which has a version 20 DCO volume.
See “DCO volume versioning” on page 68.
Log subdisks and plexes
DRL log subdisks store the dirty region log of a mirrored volume that has DRL
enabled. A volume with DRL has at least one log subdisk; multiple log subdisks
can be used to mirror the dirty region log. Each log subdisk is associated with one
Understanding Veritas Volume Manager
Dirty region logging
60