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

299Administering volume snapshots
Full-sized instant snapshots
Full-sized instant snapshots
Full-sized instant snapshots are a variation on the third-mirror volume snapshot model that
make a snapshot volume available for access as soon as the snapshot plexes have been
created. The full-sized instant volume snapshot model is illustrated in Figure 9-2.
Figure 9-2 Full-sized instant snapshot creation and usage in a backup cycle
To create an instant snapshot, you use the
vxsnap make command. This command can
either be applied to a suitably prepared empty volume that is to be used as the snapshot
volume, or it can be used to break off one or more synchronized plexes from the original
volume (which is similar to the way that the
vxassist command creates its snapshots).
Unlike a third-mirror break-off snapshot created using the
vxassist command, you can
make a backup of a full-sized instant snapshot, instantly refresh its contents from the
original volume, or attach its plexes to the original volume, without needing to completely
synchronize the snapshot plexes from the original volume.
VxVM uses a copy-on-write mechanism to ensure that the snapshot volume preserves the
contents of the original volume at the time that the snapshot is taken. Any time that the
original contents of the volume are about to be overwritten, the original data in the volume
is preserved on the snapshot volume before the write proceeds. As time goes by, and the
contents of the volume are updated, its original contents are gradually relocated to the
snapshot volume.
If desired, you can additionally select to perform either a background (non-blocking) or
foreground (blocking) synchronization of the snapshot volume. This is useful if you intend
Original
volume
vxsnap make
vxsnap dis
Independent
volume
Back up to disk, tape or other
media. The snapshot volume
can also be used to create a
replica database or file system
when synchronization is
complete.
Snapshot
volume
Start
vxsnap reattach
vxsnap prepare
or
vxsnap split
vxsnap refresh
Backup
Cycle