VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
FastResync (Fast Mirror Resynchronization)
Chapter 3114
FastResync (Fast Mirror Resynchronization)
NOTE You may need an additional license to use this feature.
The FastResync (also called Fast Mirror Resynchronization, which is
abbreviated as FMR) feature performs quick and efficient
resynchronization of stale mirrors by increasing the efficiency of the
VxVM snapshot mechanism to better support operations such as backup
and decision support. Typically, these operations require that the data
store volume is quiescent and/or secondary access to the store not affect
or impede primary access (for example,throughput, updates,consistency,
and so on).
To achieve these goals, VxVM currently provides a snapshot mechanism
that creates an exact copy of a primary volume at a particular instance
in time. After a snapshot is taken, it can be accessed independently of
the volume from which it was taken. In a shared/clustered VxVM
environment, it is possible to eliminate the resource contention and
overhead of using the snapshot simply by accessing it from a different
machine.
Fast Mirror Resynchronization overcomes certain drawbacks that exist
in an earlier version of the snapshot mechanism. They are:
• After a snapshot is taken, the primary volume and snapshot can
diverge. They are no longer consistent. As a result, the snapshot must
be discarded when it is no longer useful, and a new snapshot must be
taken in order to obtain a current copy of the primary data.
• Snapshot setup time can limit the usefulness of the snapshot feature.
This is because the period in which a snapshot can be created is
directly proportional to the size of the volume. For large, enterprise
class volumes, this period can dictate off-line policies in unacceptable
ways.
FMR Components
FMR provides two fundamental enhancements to VxVM. The first is to
optimize the mirror resynchronization process (Fast Mirror