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

337Administering volume snapshots
Creating traditional third-mirror break-off snapshots
Creating traditional third-mirror break-off snapshots
VxVM provides third-mirror break-off snapshot images of volume devices using
vxassist and other commands.
Note: To enhance the efficiency and usability of volume snapshots, turn on FastResync as
described in “Enabling FastResync on a volume” on page 284. If Persistent FastResync is
required, you must associate a version 0 DCO with the volume as described in “Adding a
version 0 DCO and DCO volume” on page 344.
You need a full license and a a Veritas FlashSnap
TM
license to use this feature.
The procedure described in this section requires a plex that is large enough to store the
complete contents of the volume. For details of a method that uses space-optimized
snapshots, see “Creating instant snapshots” on page 310.
The recommended approach to performing volume backup from the command line, or
from a script, is to use the
vxassist command. The vxassist snapstart, snapwait,
and
snapshot tasks allow you to back up volumes online with minimal disruption to
users.
The vxassist snapshot procedure consists of two steps:
■ Run vxassist snapstart to create a snapshot mirror.
■ Run vxassist snapshot to create a snapshot volume.
The
vxassist snapstart step creates a write-only backup plex which gets attached to
and synchronized with the volume. When synchronized with the volume, the backup plex
is ready to be used as a snapshot mirror. The end of the update procedure is indicated
by the new snapshot mirror changing its state to SNAPDONE. This change can be
tracked by the
vxassist snapwait task, which waits until at least one of the mirrors
changes its state to SNAPDONE. If the attach process fails, the snapshot mirror is
removed and its space is released.
Note: If the snapstart procedure is interrupted, the snapshot mirror is automatically
removed when the volume is started.
Once the snapshot mirror is synchronized, it continues being updated until it is
detached. You can then select a convenient time at which to create a snapshot volume
as an image of the existing volume. You can also ask users to refrain from using the
system during the brief time required to perform the snapshot (typically less than a
minute). The amount of time involved in creating the snapshot mirror is long in
contrast to the brief amount of time that it takes to create the snapshot volume.
The online backup procedure is completed by running the
vxassist snapshot
command on a volume with a SNAPDONE mirror. This task detaches the finished