HP Veritas Volume Manager 5.0 -UX 11i v3 Creating a shared cache object, 16k, mydg, mydg16, mydg17, Use the, snapshot, vxassist, cachevol, break-off, full-sized, regionsize, init=active

322Administering volume snapshots

Creating instant snapshots

created, and it must also have the same region size. See “Creating a volume for use as a full-sized instant or linked break-off snapshot” on page 323 for details.

Creating a shared cache objectTo create a shared cache object

1Decide on the following characteristics that you want to allocate to the cache volume that underlies the cache object:

■The size of the cache volume should be sufficient to record changes to the parent volumes during the interval between snapshot refreshes. A suggested value is 10% of the total size of the parent volumes for a refresh interval of 24 hours.

■If redundancy is a desired characteristic of the cache volume, it should be mirrored. This increases the space that is required for 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.

2Having decided on its characteristics, use the vxassist command to create the volume that is to be used for the cache volume. The following example creates a mirrored cache volume, cachevol, with size 1GB in the disk group, mydg, on the disks mydg16 and mydg17:

#vxassist -g mydg make cachevol 1g layout=mirror \ init=active mydg16 mydg17

The attribute init=active is specified to make the cache volume immediately available for use.

3Use the vxmake cache command to create a cache object on top of the cache volume that you created in the previous step:

#vxmake [-gdiskgroup] cache cache_object \ cachevolname=volume [regionsize=size] [autogrow=on] \ [highwatermark=hwmk] [autogrowby=agbvalue] \ [maxautogrow=maxagbvalue]]

If the region size, regionsize, is specified, it must be a power of 2, and be greater than or equal to 16KB (16k). If not specified, the region size of the cache is set to 64KB.