VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
FastResync
Chapter 1 53
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.
Persistent FastResync
In VxVM 3.2, Non-Persistent FastResync was augmented by the
introduction of Persistent FastResync. Unlike Non-Persistent
FastResync, Persistent FastResync keeps the FastResync maps on disk
so that they can survive both system reboots, system crashes and cluster
crashes. If Persistent FastResync is enabled on a volume or on a
snapshot volume, a data change object (DCO) and a DCO volume are
associated with the volume.
The DCO object manages information about the FastResync maps. These
maps track writes to the original volume and to each of up to 32 snapshot
volumes since the last snapshot operation. The DCO volume on disk
holds the 33 maps, each of which is 4 blocks in size by default.