VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
FastResync
Chapter 152
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 an instant in time. After a snapshot is taken,
it can be accessed 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 298.
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.