VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Volume Snapshots
Chapter 150
Volume Snapshots
VERITAS Volume Manager provides the capability for taking an image
of a volume at a given point in time. Such an image is referred to as a
volume snapshot. Such snapshots should not be confused with file
system snapshots, which are point-in-time images of a VERITAS File
System.
The figure Volume Snapshot as a Point-In-Time Image of a Volume
illustrates how a snapshot volume represents a copy of an original
volume at a given point in time. Even though the contents of the original
volume can change, the snapshot volume can be used to preserve the
contents of the original volume as they existed at an earlier time.
The snapshot volume provides a stable and independent base for making
backups of the contents of the original volume, or for other applications
such as decision support. In the figure, the contents of the snapshot
volume are eventually resynchronized with the original volume at a later
point in time.
Another possibility is to use the snapshot volume to restore the contents
of the original volume. This may be useful if the contents of the original
volume have become corrupted in some way.
NOTE If you choose to write to the snapshot volume, it may no longer be suitable
for use in restoring the contents of the original volume.
The command, vxassist snapback, can be used to return snapshot plexes
to the original volume from which they were snapped, and to
resynchronize the data in the snapshot mirrors from the data in the
original volume. This enables you to refresh the data in a snapshot after
each time that you use it to make a backup. As described in “FastResync”
on page 52, you can use the FastResync feature of VxVM to minimize the
time needed to resynchronize the data in the snapshot mirror. If
FastResync is not enabled, a full resynchronization of the data is
required.