Storage Foundation 7.4.1 Administrator's Guide - Linux

Creating instant snapshots of volume sets

Volume set names can be used in place of volume names with the following vxsnap operations on instant snapshots: addmir, dis, make, prepare, reattach, refresh, restore, rmmir, split, syncpause, syncresume, syncstart, syncstop, syncwait, and unprepare.

The procedure for creating an instant snapshot of a volume set is the same as that for a standalone volume. However, there are certain restrictions if a full-sized instant snapshot is to be created from a prepared volume set. A full-sized instant snapshot of a volume set must itself be a volume set with the same number of volumes, and the same volume sizes and index numbers as the parent. For example, if a volume set contains three volumes with sizes 1GB, 2GB and 3GB, and indexes 0, 1 and 2 respectively, then the snapshot volume set must have three volumes with the same sizes matched to the same set of index numbers. The corresponding volumes in the parent and snapshot volume sets are also subject to the same restrictions as apply between standalone volumes and their snapshots.

You can use the vxvset list command to verify that the volume sets have identical characteristics as shown in this example:

# vxvset -g mydg list vset1

VOLUME       INDEX      LENGTH      KSTATE      CONTEXT
vol_0        0          204800      ENABLED     -
vol_1        1          409600      ENABLED     -
vol_2        2          614400      ENABLED     -

# vxvset -g mydg list snapvset1

VOLUME       INDEX      LENGTH      KSTATE      CONTEXT
svol_0       0          204800      ENABLED     -
svol_1       1          409600      ENABLED     -
svol_2       2          614400      ENABLED     -

A full-sized instant snapshot of a volume set can be created using a prepared volume set in which each volume is the same size as the corresponding volume in the parent volume set. Alternatively, you can use the nmirrors attribute to specify the number of plexes that are to be broken off provided that sufficient plexes exist for each volume in the volume set.

The following example shows how to prepare a source volume set, vset1, and an identical volume set, snapvset1, which is then used to create the snapshot:

# vxsnap -g mydg prepare vset1
# vxsnap -g mydg prepare snapvset1
# vxsnap -g mydg make source=vset1/snapvol=snapvset1

To create a full-sized third-mirror break-off snapshot, you must ensure that each volume in the source volume set contains sufficient plexes. The following example shows how to achieve this by using the vxsnap command to add the required number of plexes before breaking off the snapshot:

# vxsnap -g mydg prepare vset2
# vxsnap -g mydg addmir vset2 nmirror=1
# vxsnap -g mydg make source=vset2/newvol=snapvset2/nmirror=1

To create a space-optimized instant snapshot of a volume set, the commands are again identical to those for a standalone volume as shown in these examples:

# vxsnap -g mydg prepare vset3
# vxsnap -g mydg make source=vset3/newvol=snapvset3/cachesize=20m

# vxsnap -g mydg prepare vset4
# vxsnap -g mydg make source=vset4/newvol=snapvset4/cache=mycobj

Here a new cache object is created for the volume set, vset3, and an existing cache object, mycobj, is used for vset4.