Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

60 Understanding Veritas Volume Manager
Volu m e snap s hots
Figure 1-31 Volume snapshot as a point-in-time image of a volume
The traditional type of volume snapshot in VxVM is of 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. For more information about this type of
snapshot, see “Traditional third-mirror break-off snapshots” on page 297.
The FastResync feature was introduced to track writes to the original volume. This
tracking means that only a partial, and therefore much faster, resynchronization is required
on reattaching the snapshot plex. In later releases, the snapshot model was enhanced to
allow snapshot volumes to contain more than a single plex, reattachment of a subset of a
snapshot volume’s plexes, and persistence of FastResync across system reboots or cluster
restarts.
For more information about FastResync, see “FastResync” on page 62.
Release 4.0 of VxVM introduced full-sized instant snapshots and space-optimized instant
snapshots, which offer advantages over traditional third-mirror snapshots such as
immediate availability and easier configuration and administration. You can also use the
third-mirror break-off usage model with full-sized snapshots, where this is necessary for
write-intensive applications.
Snapshot
volume
Snapshot
volume
Original
volume
Original
volume
Original
volume
Original
volume
Snapshot volume is
created at time T2
Snapshot
volume
Time
T1
T2
T3
T4
Resynchronize
snapshot volume
from original volume
Snapshot volume retains
image taken at time T2
Snapshot volume is
updated at time T4