VERITAS Storage Foundation 4.1 Intelligent Storage Provisioning Administrator's Guide

Chapter 5, Administering Instant Snapshots

Creating a Shared Cache Volume and Preparing Space-Optimized Snapshots

◆ If redundancy is a desired characteristic of the cache volume, it should be mirrored. This

increases the space required by the cache volume in proportion to the number of mirrors

that it has.

◆ If the cache volume is mirrored, space is required on at least as many disks as it has

mirrors. These disks should not be shared with the disks used for the parent volumes. The

disks should also be chosen to avoid impacting I/O performance for critical volumes, or

hindering disk group split and join operations.

2. Having decided on its characteristics, use the vxvoladm command to create the cache

volume:

# vxvoladm [-g diskgroup] [-p storage_pool] make cachevol size \

[storage_specification ...] [attribute ...] [regionsize size] \

type=cachevolume

The attribute regionsize specifies the minimum size of each chunk (or region) of a volume

whose contents are tracked for changes. The region size must be a power of 2, and be greater

than or equal to 16KB. A smaller value requires more disk space for the change maps, but the

finer granularity provides faster resynchronization. The default region size is 64k (64KB).

The following example creates a 1GB mirrored cache volume, cachevol, in the clone

storage pool, myclpool, within the disk group, mydg:

# vxvoladm -g mydg -p myclpool make mycache 1g \

rules="mirror 2" type=cachevolume

3. Once the cache volume has been created, use the following command to prepare each

space-optimized snapshot that uses the cache volume:

# vxvoladm [-g diskgroup] [-p storage_pool] make sovol srcvol_len\

[storage_specification ...] [attribute ...] type=snapshot \

cachevolume=cachevol init=active

Note The srcvol_len argument specifies the length of the source volume for which the

snapshot is being prepared. This value defines the logical size of the snapshot. The

actual amount of storage that the snapshot requires is less than this, and is limited by the

size of the cache volume.

For example, this command creates a space-optimized snapshot volume that uses the cache

volume, mycache:

# vxvoladm -g mydg -p myclpool make mysovol 10g type=snapshot \

cachevolume=mycache

Note The argument 10g is the size of the source volume for which the snapshot mysovol is

being prepared.