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

Chapter 1, Understanding VERITAS Volume Manager
Volume Resynchronization
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VxVM needs to ensure that all mirrorscontain exactly the same data andthat thedata and
parity in RAID-5 volumes agree. This process is called volume resynchronization. For
volumes thatare part ofdisk groupsthat are automaticallyimported at boot time(such as
rootdg), the resynchronization process takes place when the system reboots.
Not all volumes require resynchronizationafter asystem failure. Volumes thatwere never
written or that were quiescent (that is, had no active I/O) when the system failure
occurred could not have had outstanding writes and do not require resynchronization.
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 when the system reboots require resynchronization.
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)” and “FastResync,” for more information.
For volumes usedby databaseapplications, theSmartSync™ RecoveryAccelerator canbe
used (see “SmartSync Recovery Accelerator” on page 49).