Preparing a Volume for Instant Snapshot Operations

To prepare a volume for instant snapshot operations, a version 20 Data Change Object (DCO) and DCO volume must first be associated with that volume.

To add a version 20 DCO object and DCO volume to an existing volume, use the following procedure:

1. Ensure that the disk group containing the existing volume has been upgraded to at least version 110. Use the following command to check the version of a disk group:
   # vxprint -l diskgroup | egrep 'version:'

   To upgrade a disk group, use the following command:

   
   # vxdg upgrade diskgroup
2. Use the following command to add a version 20 DCO and DCO volume to an existing volume:
   # vxsnap [-g diskgroup] prepare volume [ndcomirs=number] \ 
      [regionsize=size] [alloc=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.

   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.

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     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.
   
   By default, a new-style 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.

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3. If you are going to create a snapshot volume by breaking off existing plexes, use the following command to add one or more snapshot mirrors to the volume:
   # vxsnap [-b] [-g diskgroup] addmir volume [nmirror=N] \
   [alloc=storage_attribute[,...]]

   By default, one snapshot plex is added unless you specify a number using the nmirror attribute. For a backup, you should usually only require one plex. The mirrors remain in the SNAPATT state until they are fully synchronized. The -b option can be used to perform the synchronization in the background. Once synchronized, the mirrors are placed in the SNAPDONE state.

   For example, the following command adds 2 mirrors to the volume, vol1, on disks mydg10 and mydg11:

   
   # vxsnap -g mydg addmir vol1 nmirror=2 alloc=mydg10,mydg11

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     Note    Do not perform this step if you create a full-sized snapshot volume using a suitably prepared empty volume (see Creating a Volume for Use as a Full-Sized Instant Snapshot), or if you create space-optimized snapshots that use a cache (see Creating a Shared Cache Object).

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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 DCO volume with plexes on disk05 and disk06, and a region size of 32KB, to the volume, myvol, use the following command:

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 for volume vol1 from disk03 and disk04 to disk07 and disk08:

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 zoo (the TUTIL0 and PUTIL0 columns are omitted for clarity):

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

For more information, see Considerations for Placing DCO Plexes, and the vxassist(1M) and vxsnap(1M) manual pages.

Considerations for Placing DCO Plexes

If you use the vxassist command or the VERITAS Enterprise Administrator (VEA) to create both a volume and its DCO, or the vxsnap prepare command to add a DCO to a volume, the DCO plexes are automatically placed on different disks from the data plexes of the parent volume. In previous releases, version 0 DCO plexes were placed on the same disks as the data plexes for convenience when performing disk group split and move operations. As the version 20 DCOs in VxVM 4.0 and later releases support dirty region logging (DRL) in addition to Persistent FastResync, it is preferable for the DCO plexes to be separated from the data plexes. This improves the I/O performance of the volume, and provides resilience for the DRL logs.

If you use the vxsnap prepare command to set up a DCO, you must ensure that the disks that contain the plexes of the DCO volume accompany their parent volume during the move. Use the vxprint command on a volume to examine the configuration of its associated DCO volume.

Examples of Disk Groups That Can and Cannot be Split illustrates some instances in which it is not be possible to split a disk group because of the location of the DCO plexes.

For more information about relocating DCO plexes, see Preparing a Volume for Instant Snapshot Operations.

Examples of Disk Groups That Can and Cannot be Split

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