Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Figure 1-31
Volume snapshot as a point-in-time image of a volume
Snapshot volume is created
at time T2
Snapshot volume retains
image taken at time T2
Snapshot volume is updated
at time T4
Resynchronize snapshot volume
from the original volume
T1
T2
Original volume
Original volume Snapshot volume
Original volume
Snapshot volume
Original volume
Snapshot volume
T4
T3
Time
Even though the contents of the original volume can change, the snapshot volume
preserves 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.
Warning: If you write to the snapshot volume, it may no longer be suitable for use
in restoring the contents of the original volume.
One type of volume snapshot in VxVM is the third-mirror break-off type. This
name comes from its implementation where a snapshot plex (or third mirror) is
added to a mirrored volume. The contents of the snapshot plex are then
synchronized from the original plexes of the volume. When this synchronization
is complete, the snapshot plex can be detached as a snapshot volume for use in
backup or decision support applications. At a later time, the snapshot plex can be
reattached to the original volume, requiring a full resynchronization of the
snapshot plex’s contents.
The FastResync feature was introduced to track writes to the original volume.
This tracking means that only a partial, and therefore much faster,
61Understanding Veritas Volume Manager
Volume snapshots