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

Creating Instant Snapshots
280 VERITAS Volume Manager Administrator’s Guide
The second form of the vxsnap make command uses one of the following attributes
to create the new snapshot volume, snapvol, by breaking off one or more existing
plexes in the original volume:
plex Specifies the plexes in the existing volume that are to be broken off. This
attribute can only be used with plexes that are in the ACTIVE state.
nmirror Specifies how many plexes are to be broken off. This attribute can only be
used with plexes that are in the SNAPDONE state. (That is, they have been
added to the volume by using the vxsnap addmir command.)
Snapshots that are created from one or more ACTIVE or SNAPDONE plexes in the
volume are already synchronized by definition.
For backup purposes, a snapshot volume with one plex should be sufficient.
Note Break-off snapshots are suitable for write-intensive volumes (such as for
database redo logs) where the copy-on-write mechanism of space-optimized or
full-sized instant snapshots might degrade the performance of the volume.
Snapshots that you create by using vxsnap to detach existing plexes in the
volume are functionally equivalent to the traditional third-mirror break-off
snapshots that you can create by using vxassist. As future releases may not
support this functionality in vxassist, it is recommended that you use
vxsnap to create this type of snapshot.
For example, to create the instant snapshot volume, snap2myvol, of the volume,
myvol, in the disk group, mydg, from a single existing plex in the volume, use the
following command:
# vxsnap -g mydg make source=myvol/newvol=snap2myvol/nmirror=1
The next example shows how to create a mirrored snapshot from two existing plexes
in the volume:
# vxsnap -g mydg make source=myvol/newvol=snap2myvol\
/plex=myvol-03,myvol-04
❖ To create a space-optimized instant snapshot, snapvol, that uses a named shared
cache object:
# vxsnap [-g diskgroup] make source=vol/newvol=snapvol\
/[cache=cacheobject] [alloc=storage_attributes]
For example, to create the space-optimized instant snapshot, snap3myvol, of the
volume, myvol, in the disk group, mydg, on the disk mydg14, and which uses the
shared cache object, cobjmydg, use the following command:
# vxsnap -g mydg make source=myvol/newvol=snap3myvol\
/cache=cobjmydg alloc=mydg14