Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 9, Administering Volume Snapshots
Creating Traditional Third-Mirror Break-Off Snapshots
299
Creating Traditional Third-Mirror Break-Off Snapshots
VxVM provides third-mirror break-off snapshot images of volume devices using
vxassist and other commands.
Note It is recommended that you use the instant snapshot mechanism for backup.
Support for traditional third-mirror break-off snapshots created using the
vxassist command may be removed in a future release. If you are currently using
such snapshots, you should consider migrating your backup strategy to use
snapshots that have been created using the vxsnap command.
To enhance the efficiency and usability of volume snapshots, turn on FastResync as
described in “Enabling FastResync on a Volume” on page 252. 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 307.
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 275.
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:
1. Run vxassist snapstart to create a snapshot mirror.
2. 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.