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Creating and managing linked break-off snapshot volumes

Linked break-off snapshots are the only form of volume snapshot that can be used to implement offhost processing for ISP volumes as described in the Veritas Storage Foundation Intelligent Storage Provisioning Solutions Guide. They are also the most suitable type of snapshot to use with write-intensive volumes, such as database redo logs.

For linked break-off snapshots, you must prepare a volume that is to be used as the snapshot volume. This must be the same size as the volume for which the snapshot is being 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.

The attributes for a snapshot are specified as a tuple to the vxsnap make command. This command accepts multiple tuples; one for each snapshot that is being created. Each element of a tuple is separated from the next by a slash character (/). Tuples are separated by white space.

 To create and manage a linked break-off snapshot

1. Use the following command to link the prepared snapshot volume, snapvol, to the data volume:

   # vxsnap [-g diskgroup] [-b] addmir volume mirvol=snapvol \

     [mirdg=snapdg]

   The optional mirdg attribute can be used to specify the snapshot volume's current disk group, snapdg. The -b option can be used to perform the synchronization in the background. If the -b option is not specified, the command does not return until the link becomes ACTIVE.

   For example, the following command links the prepared volume, prepsnap, in the disk group, mysnapdg, to the volume, vol1, in the disk group, mydg:

   # vxsnap -g mydg -b addmir vol1 mirvol=prepsnap \

     mirdg=mysnapdg

   After the -b option is specified, you can use the vxsnap snapwait in the next command to wait for the synchronization of the snapshot plexes or linked snapshot volume to complete, as shown in the following example:

   # vxsnap -g mydg snapwait vol1 mirvol=prepsnap \

     mirdg=mysnapvoldg

2. To create a linked break-off snapshot, use the following form of the vxsnap make command:

   # vxsnap [-g diskgroup] make source=volume/snapvol=snapvol\

   [/snapdg=snapdiskgroup]

The snapdg attribute must be used to specify the snapshot volume's disk group if this is different from that of the data volume.

For example, to use the prepared volume, prepsnap, as the snapshot for the volume, vol1, in the disk group, mydg, use the following command:

# vxsnap -g mydg make \

  source=vol1/snapvol=prepsnap/snapdg=mysnapdg

3. Use fsck (or some utility appropriate for the application running on the volume) to clean the temporary volume's contents.

   For example, you can use the following command with a VxFS file system:

   # fsck -F vxfs /dev/vx/rdsk/diskgroup/snapshot

4. If you require a backup of the data in the snapshot, use an appropriate utility or operating system command to copy the contents of the snapshot to tape, or to some other backup medium.
5. You now have the following choices of what to do with a linked break-off snapshot:
   • Refresh the contents of the snapshot. This creates a new point-in-time image of the original volume ready for another backup. If synchronization was already in progress on the snapshot, this operation may result in large portions of the snapshot having to be resynchronized.

     See Refreshing an instant snapshot.

     This operation is not possible if the linked volume and snapshot are in different disk groups.

   • Relink the snapshot volume with the original volume.

     See Reattaching a linked break-off snapshot volume.

   • Dissociate the snapshot volume entirely from the original volume. This may be useful if you want to use the copy for other purposes such as testing or report generation. If desired, you can delete the dissociated volume.

     See Dissociating an instant snapshot.

   • If the snapshot is part of a snapshot hierarchy, you can also choose to split this hierarchy from its parent volumes.

     See Splitting an instant snapshot hierarchy.