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Creating and managing space-optimized instant snapshots

Space-optimized instant snapshots are not suitable for write-intensive volumes (such as for database redo logs) because the copy-on-write mechanism may degrade the performance of the volume.

If you intend to split the volume and snapshot into separate disk groups (for example, to perform off-host processing), you must use a fully synchronized full-sized instant, third-mirror break-off or linked break-off snapshot (which do not require a cache object). You cannot use a space-optimized instant snapshot for this purpose.

Creation of space-optimized snapshots that use a shared cache fails if the region size specified for the volume is smaller than the region size set on the cache.

If the region size of a space-optimized snapshot differs from the region size of the cache, this can degrade the system's performance compared to the case where the region sizes are the same.

For space-optimized instant snapshots that share a cache object, the specified region size must be greater than or equal to the region size specified for the cache object.

See "Creating a shared cache object" on page 330.

The attributes for a snapshot are specified as a tuple to the vxsnap make command. This command accepts multiple tuples. One tuple is required 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 space-optimized instant snapshot

1. Use the vxsnap make command to create a space-optimized instant snapshot. This snapshot can be created by using an existing cache object in the disk group, or a new cache object can be created for its use.
2. 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

See "Creating a shared cache object" on page 330.

• To create a space-optimized instant snapshot, snapvol, and also create a cache object for it to use:

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

  [/cachesize=size][/autogrow=yes][/ncachemirror=number]\

  [alloc=storage_attributes]

  The cachesize attribute determines the size of the cache relative to the size of the volume. The autogrow attribute determines whether VxVM will automatically enlarge the cache if it is in danger of overflowing. By default, the cache is not grown.

  If autogrow is enabled, but the cache cannot be grown, VxVM disables the oldest and largest snapshot that is using the same cache, and releases its cache space for use.

  The ncachemirror attribute specifies the number of mirrors to create in the cache volume. For backup purposes, the default value of 1 should be sufficient.

  For example, to create the space-optimized instant snapshot, snap4myvol, of the volume, myvol, in the disk group, mydg, on the disk mydg15, and which uses a newly allocated cache object that is 1GB in size, but which can automatically grow in size, use the following command:

  # vxsnap -g mydg make source=myvol/new=snap4myvol\

  /cachesize=1g/autogrow=yes alloc=mydg15

If a cache is created implicitly by specifying cachesize, and ncachemirror is specified to be greater than 1, a DCO is attached to the cache volume to enable dirty region logging (DRL). DRL allows fast recovery of the cache backing store after a system crash. The DCO is allocated on the same disks as those that are occupied by the DCO of the source volume. This is done to allow the cache and the source volume to remain in the same disk group for disk group move, split and join operations.

2. 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 this command with a VxFS file system:

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

The specified device must have a valid entry in the /etc/vfstab file.

3. 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.
4. You now have the following choices of what to do with a space-optimized instant 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" on page 345.

   • Restore the contents of the original volume from the snapshot volume. The space-optimized instant snapshot remains intact at the end of the operation.

     See "Restoring a volume from an instant snapshot" on page 347.