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Preparing a volume for DRL and instant snapshots

  Note   This procedure describes how to add a version 20 data change object (DCO) and DCO volume to a volume that you previously created in a disk group with a version number of 110 or greater. If you are creating a new volume in a disk group with a version number of 110 or greater, you can specify the co-creation of a DCO and DCO volume and enable DRL as described in Creating a volume with a version 20 DCO volume. If the volume was created in a release prior to VxVM 4.0, use the procedure in Upgrading existing volumes to use version 20 DCOs.

You need a full VxVM license and a Veritas FlashSnap^TM or FastResync license to use the DRL and FastResync features. Even if you do not have a license, you can configure a DCO object and DCO volume so that snap objects are associated with the original and snapshot volumes. For more information about snap objects, see How persistent FastResync works with snapshots. See Determining the DCO version number for details of how to determine the version number of a volume's DCO.

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

# vxsnap [-g diskgroup] prepare volume [ndcomirs=number] \

  [regionsize=size] [drl=on|sequential|off] \

  [storage_attribute ...]

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 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.

  Note   The vxsnap prepare command automatically enables Persistent FastResync on the volume. Persistent FastResync is also set automatically on any snapshots that are generated from a volume on which this feature is enabled.

If the volume is a RAID-5 volume, it is converted to a layered volume that can be used with instant snapshots and Persistent FastResync. See Using a DCO and DCO volume with a RAID-5 volume for details.

By default, a version 20 DCO volume contains 32 per-volume maps. If you require more maps than this, you can use the vxsnap addmap command to add more maps. See the vxsnap(1M) manual page for details of this command.