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

310 Administering volume snapshots
Creating instant snapshots
Creating instant snapshots
Note: You need a full license and a a Veritas FlashSnap
TM
license to use this feature.
VxVM allows you to make instant snapshots of volumes by using the
vxsnap command.
Note: The information in this section also applies to RAID-5 volumes that have been
converted to a special layered volume layout by the addition of a DCO and DCO volume.
See “Using a DCO and DCO volume with a RAID-5 volume” on page 269 for details.
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.
The vxsnap prepare step creates a DCO and DCO volume and associates this with the
volume. It also enables Persistent FastResync on the volume.
The vxsnap make step creates an instant snapshot that is immediately available for
making a backup. After the snapshot has been taken, read requests for data in the original
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 the contents of the original volume at the point in time that the snapshot
was taken. If you wish to move an instant snapshot volume to another disk group for
export to another machine for off-host processing, or to turn it into an independent
volume, you must ensure that the snapshot volume has been completely synchronized.
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, you must wait for the new resynchronization to complete.
You can create instant snapshots of volume sets by replacing volume names with volume
set names in the
vxsnap command. For more information, see “Creating instant snapshots
of volume sets” on page 324.