Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Dirty flags
VxVM records when a volume is first written to and marks it as dirty. When a
volume is closed by all processes or stopped cleanly by the administrator, and all
writes have been completed, VxVM removes the dirty flag for the volume. Only
volumes that are marked dirty require resynchronization.
Resynchronization process
The process of resynchronization depends on the type of volume. For mirrored
volumes, resynchronization is done by placing the volume in recovery mode (also
called read-writeback recovery mode). Resynchronization of data in the volume
is done in the background. This allows the volume to be available for use while
recovery is taking place. RAID-5 volumes that contain RAID-5 logs can “replay”
those logs. If no logs are available, the volume is placed in reconstruct-recovery
mode and all parity is regenerated.
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 57.
See “FastResync” on page 63.
For mirrored volumes used by Oracle, you can use the SmartSync feature, which
further improves performance.
See “SmartSync recovery accelerator” on page 59.
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.
57Understanding Veritas Volume Manager
Dirty region logging