VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Administering Volumes
Backing up Volumes Online
Chapter 8 295
NOTE You may need an additional license to use this feature.
VxVM provides snapshot images of volume devices using vxassist and
other commands. If the fsgen volume usage type is set on a volume that
contains a VERITAS File System (VxFS), the snapshot mechanism
ensures the internal consistency of the file system that is backed up. For
file system types, there may be inconsistencies between in-memory data
and the data in the snapshot image.
There are various procedures for doing backups, depending upon the
requirements for integrity of the volume contents. The procedures
require a plex that is large enough to store the complete contents of the
volume. The plex can be larger than necessary, but if a plex that is too
small is used, an incomplete copy results.
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:
Step 1. Running vxassist snapstart to create a snapshot mirror
Step 2. Running 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.
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.