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

Chapter 1, Understanding VERITAS Volume Manager
FastResync
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Alternatively, you can use the vxassist snapclear command to break the association
between the original volume and the snapshot volume. The snapshot volume then has an
existence that isindependent ofthe original volume.This is useful forapplications thatdo
not require the snapshot to be resynchronized with the original volume.
For more information about taking snapshots of a volume, see “Backing Up Volumes
Online Using Snapshots” on page 214, and the vxassist(1M) manual page.
FastResync
Note You may need an additional license to use this feature.
The FastResync feature (previously called fast mirror resynchronization or FMR)
performs quick and efficient resynchronization of stale mirrors (a mirror that is not
synchronized). This increases the efficiency of the VxVM snapshot mechanism, and
improves the performance of operations such as backup and decision support
applications. Typically, these operations require that the volume is quiescent, and that
they are not impeded by updates to the volume by other activities on the system. To
achieve these goals, the snapshot mechanism in VxVM creates an exact copy of a primary
volume at aninstant in time.After a snapshot istaken, it can beaccessed independently of
the volume from which it was taken. In a clustered VxVM environment with shared
access to storage, it is possible to eliminate the resource contention and performance
overhead of using a snapshot simply by accessing it from a different node.
For details of how to enable FastResync on a per-volume basis, see “Enabling FastResync
on a Volume” on page 207.
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
returningmirror is a mirror that was previouslydetached andis inthe 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.