Preparing a Volume for DRL and Instant Snapshots

Use the following command to add a version 20 DCO and DCO volume to a volume:

The ndcomirs attribute specifies the number of DCO plexes that are created in the DCO volume. It is recommended that you configure as many DCO plexes as there are data and snapshot plexes in the volume. The DCO plexes are used to set up a DCO volume for any snapshot volume that you subsequently create from the snapshot plexes. For example, specify ndcomirs=5 for a volume with 3 data plexes and 2 snapshot plexes.

The value of the regionsize attribute specifies the size of the tracked regions in the volume. A write to a region is tracked by setting a bit in the change map. The default value is 64k (64KB). A smaller value requires more disk space for the change maps, but the finer granularity provides faster resynchronization.

To enable DRL logging on the volume, specify drl=on (this is the default setting). If sequential DRL is required, specify drl=sequential. If DRL is not required, specify drl=off.

You can also specify vxassist-style storage attributes to define the disks that can and/or cannot be used for the plexes of the DCO volume. See Specifying Storage for Version 20 DCO Plexes for details.

Specifying Storage for Version 20 DCO Plexes

If the disks that contain volumes and their snapshots are to be moved into different disk groups, you must ensure that the disks that contain their DCO plexes can accompany them. You can use storage attributes to specify which disks to use for the DCO plexes. (If you do not want to use dirty region logging (DRL) with a volume, you can specify the same disks as those on which the volume is configured, assuming that space is available on the disks). For example, to add a DCO object and mirrored DCO volume with plexes on disk05 and disk06 to the volume, myvol, use the following command:

To view the details of the DCO object and DCO volume that are associated with a volume, use the vxprint command. The following is example vxprint -vh output for the volume named vol1 (the TUTIL0 and PUTIL0 columns are omitted for clarity):

In this output, the DCO object is shown as vol1_dco, and the DCO volume as vol1_dcl with 2 plexes, vol1_dcl-01 and vol1_dcl-02.

If required, you can use the vxassist move command to relocate DCO plexes to different disks. For example, the following command moves the plexes of the DCO volume, vol1_dcl, for volume vol1 from disk03 and disk04 to disk07 and disk08:

For more information, see Moving DCO Volumes Between Disk Groups, and the vxassist(1M) and vxsnap(1M) manual pages.

Using a DCO and DCO Volume with a RAID-5 Volume

The procedure in the previous section can be used to add a DCO and DCO volume to a RAID-5 volume. This allows you to use Persistent FastResync on the volume for fast resynchronization of snapshots on returning them to their original volume. However, the procedure has the side effect of converting the RAID-5 volume into a special type of layered volume. You can create space-optimized instant snapshots of such a volume, and you can add mirrors that may be broken off as full-sized instant snapshots. You cannot relayout or resize such a volume unless you convert it back to a pure RAID-5 volume.

To convert a volume back to a RAID-5 volume, remove any snapshot plexes from the volume, and dissociate the DCO and DCO volume from the layered volume using the procedure described in Removing Support for DRL and Instant Snapshots from a Volume. You can then perform relayout and resize operations on the resulting non-layered RAID-5 volume.

To allow Persistent FastResync to be used with the RAID-5 volume again, re-associate the DCO and DCO volume as described in Preparing a Volume for DRL and Instant Snapshots.

Determining the DCO Version Number

The instant snapshot and DRL-enabled DCO features require that a version 20 DCO be associated with a volume, rather than an earlier version 0 DCO.

To find out the version number of a DCO that is associated with a volume:

1. Use the vxprint command on the volume to discover the name of its DCO:
   # DCONAME='vxprint [-g diskgroup] -F%dco_name volume'
2. Use the vxprint command on the DCO to determine its version number:
   # vxprint [-g diskgroup] -F%version $DCONAME

Determining if DRL is Enabled on a Volume

To determine if DRL (configured using a version 20 DCO volume) is enabled on a volume:

1. Use the vxprint command on the volume to discover the name of its DCO:
   # DCONAME='vxprint [-g diskgroup] -F%dco_name volume'
2. To determine if DRL is enabled on the volume, use the following command with the volume's DCO:
   # vxprint [-g diskgroup] -F%drl $DCONAME

   DRL is enabled if this command displays on.

3. If DRL is enabled, use this command with the DCO to determine if sequential DRL is enabled:
   # vxprint [-g diskgroup] -F%sequentialdrl $DCONAME

   Sequential DRL is enabled if this command displays on.

Alternatively, you can use this command with the volume:

This displays the logging type as REGION for DRL, DRLSEQ for sequential DRL, or NONE if DRL is not enabled.

Determining if DRL Logging is Active on a Volume

To determine if DRL logging is active on a mirrored volume:

1. Use the following vxprint commands to discover the name of the volume's DCO volume:
   # DCONAME='vxprint [-g diskgroup] -F%dco_name volume'
   # DCOVOL='vxprint [-g diskgroup] -F%parent_vol $DCONAME'
2. Use the vxprint command on the DCO volume to find out if DRL logging is active:
   # vxprint [-g diskgroup] -F%drllogging $DCOVOL

   This command returns on if DRL logging is enabled.

Disabling and Re-enabling DRL

To disable DRL (configured using a version 20 DCO volume) on a volume, enter the following command:

To re-enable DRL on a volume, enter this command:

To re-enable sequential DRL on a volume, enter:

You can use these commands to change the DRL policy on a volume by first disabling and then re-enabling DRL as required. DRL is automatically disabled if a data change map (DCM, used with VERITAS Volume Replicator) is attached to a volume.

Removing Support for DRL and Instant Snapshots from a Volume

To remove support for DRL and instant snapshot operation from a volume, use the following command to remove the DCO and DCO volume that are associated with the volume:

This command also has the effect of disabling FastResync tracking on the volume.