Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

63Understanding Veritas Volume Manager
FastResync
FastResync enhancements
FastResync provides two fundamental enhancements to VxVM:
■ FastResync optimizes mirror resynchronization by keeping track of updates to stored
data that have been missed by a mirror. (A mirror may be unavailable because it has
been detached from its volume, either automatically by VxVM as the result of an
error, or directly by an administrator using a utility such as
vxplex or vxassist. A
returning mirror is a mirror that was previously detached and is in the process of
being re-attached to its original volume as the result of the
vxrecover or vxplex
att operation.) When a mirror returns to service, only the updates that it has missed
need to be re-applied to resynchronize it. This requires much less effort than the
traditional method of copying all the stored data to the returning mirror.
Once FastResync has been enabled on a volume, it does not alter how you administer
mirrors. The only visible effect is that repair operations conclude more quickly.
■ FastResync allows you to refresh and re-use snapshots rather than discard them. You
can quickly re-associate (snapback) snapshot plexes with their original volumes. This
reduces the system overhead required to perform cyclical operations such as backups
that rely on the snapshot functionality of VxVM.
Non-persistent FastResync
Non-persistent FastResync allocates its change maps in memory. If non-persistent
FastResync is enabled, a separate FastResync map is kept for the original volume and for
each snapshot volume. Unlike a dirty region log (DRL), they do not reside on disk nor in
persistent store. This has the advantage that updates to the FastResync map have little
impact on I/O performance, as no disk updates needed to be performed. However, if a
system is rebooted, the information in the map is lost, so a full resynchronization is
required on snapback. This limitation can be overcome for volumes in cluster-shareable
disk groups, provided that at least one of the nodes in the cluster remained running to
preserve the FastResync map in its memory. However, a node crash in a High Availability
(HA) environment requires the full resynchronization of a mirror when it is reattached to
its parent volume.
How non-persistent FastResync works with snapshots
The snapshot feature of VxVM takes advantage of FastResync change tracking to record
updates to the original volume after a snapshot plex is created. After a snapshot is taken,
the
snapback option is used to reattach the snapshot plex. Provided that FastResync is
enabled on a volume before the snapshot is taken, and that it is not disabled at any time
before the snapshot is reattached, the changes that FastResync records are used to
resynchronize the volume during the snapback. This considerably reduces the time needed
to resynchronize the volume.