VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Backing up Volumes Online
214 VERITAS Volume Manager Administrator’s Guide
Backing Up Volumes Online Using Snapshots
Note You canuse the proceduredescribed in this section tocreate a snapshot ofa RAID-5
volume and to back it up.
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 hfs and other 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:
1. Running vxassist snapstart to create a snapshot mirror
2. Running vxassist snapshot to create a snapshot volume
The vxassist snapstart step creates a write-only backup plex which gets attached to
andsynchronizedwith the volume.Whensynchronizedwiththe volume,the backupplex
is ready to beused as asnapshot mirror. The endof the updateprocedureis indicatedby
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 thenselect a convenient time at which tocreate 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.