VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Volume Resynchronization
Chapter 1 45
Resynchronization Process
The process of resynchronization depends on the type of volume. 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. 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.
The process of 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 addressed
by using dirty region logging (DRL) and FastResync (fast mirror
resynchronization) for mirrored volumes, or by ensuring that RAID-5
volumes have valid RAID-5 logs. See the following sections, “Dirty
Region Logging (DRL)” on page 46 and “FastResync” on page 52 for more
information.
For volumes used by database applications, the SmartSync™ Recovery
Accelerator can be used (see “SmartSync Recovery Accelerator” on
page 48).