Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

from an instant snapshot. The volume that is used to restore the original volume
can either be a true backup of the original's contents, or it may have been modified
in some way (for example, by applying a database log replay or by running a file
system checking utility such as fsck). All synchronization of the contents of this
backup must be completed before the original volume can be restored from it.
The original volume is immediately available for use while its contents are being
restored.
See “Restoring a volume from an instant snapshot” on page 235.
Creating instant snapshots
VxVM allows you to make instant snapshots by using the vxsnap command.
You can also take instant snapshots of RAID-5 volumes that have been converted
to a special layered volume layout by the addition of a DCO and DCO volume.
A plex in a full-sized instant snapshot requires as much space as the original
volume. If you instead make a space-optimized instant snapshot of a volume, this
only requires enough storage to record the original contents of the parent volume
as they are changed during the life of the snapshot.
The recommended approach to performing volume backup from the command
line, or from a script, is to use the vxsnap command. The vxsnap prepare and
make tasks allow you to back up volumes online with minimal disruption to users.
vxsnap prepare creates a DCO and DCO volume and associates this with the
original volume. It also enables Persistent FastResync.
vxsnap make creates an instant snapshot that is immediately available for making
a backup. After the snapshot has been taken, read requests for data in the instant
snapshot volume are satisfied by reading either from a non-updated region of the
original volume, or from the copy of the original contents of an updated region
that have been recorded by the snapshot.
Note: Synchronization of a full-sized instant snapshot from the original volume
is enabled by default. If you specify the syncing=no attribute to vxsnap make, this
disables synchronization, and the contents of the instant snapshot are unlikely
ever to become fully synchronized with those of the original volume at the point
in time that the snapshot was taken. In such a case, the snapshot cannot be used
for off-host processing, nor can it become an independent volume.
You can immediately retake a full-sized or space-optimized instant snapshot at
any time by using the vxsnap refresh command. If a fully synchronized instant
snapshot is required, the new resynchronization must first complete.
Administering volume snapshots
Creating instant snapshots
212