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

FastResync
44 VERITAS Volume Manager Administrator’s Guide
◆ 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, introduced in VxVM 3.1, 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 FastResyncwas augmentedby theintroduction 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.
Persistent FastResync can also track the association between volumes and their snapshot
volumes after they are moved into different disk groups. When the disk groups are
rejoined, this allows the snapshot plexes to be quickly resynchronized. This ability is not
supported by Non-Persistent FastResync. See “Reorganizing the Contents of Disk
Groups” on page 121 for details.