Adding a Version 0 DCO and DCO Volume

To put Persistent FastResync into effect for a volume, a Data Change Object (DCO) and DCO volume must first be associated with that volume. When you have added a DCO object and DCO volume to a volume, you can then enable Persistent FastResync on the volume as described in Enabling FastResync on a Volume.

To add a DCO object and DCO volume to an existing volume (which may already have dirty region logging (DRL) enabled), use the following procedure:

1. Ensure that the disk group containing the existing volume has been upgraded to at least version 90. Use the following command to check the version of a disk group:
   # vxdg list diskgroup

   To upgrade a disk group to the latest version, use the following command:

   
   # vxdg upgrade diskgroup

   For more information, see Upgrading a Disk Group.

2. Use the following command to turn off Non-Persistent FastResync on the original volume if it is currently enabled:
   # vxvol [-g diskgroup] set fastresync=off volume

   If you are uncertain about which volumes have Non-Persistent FastResync enabled, use the following command to obtain a listing of such volumes:

   
   # vxprint [-g diskgroup] -F "%name" \
      -e "v_fastresync=on && !v_hasdcolog"
3. Use the following command to add a DCO and DCO volume to the existing volume:
   # vxassist [-g diskgroup] addlog volume logtype=dco \
     [ndcomirror=number] [dcolen=size] [storage_attributes]

   For non-layered volumes, the default number of plexes in the mirrored DCO volume is equal to the lesser of the number of plexes in the data volume or 2. For layered volumes, the default number of DCO plexes is always 2. If required, use the ndcomirror attribute to specify a different number. It is recommended that you configure as many DCO plexes as there are existing data and snapshot plexes in the volume. For example, specify ndcomirror=3 when adding a DCO to a 3-way mirrored volume.

   The default size of each plex is 132 blocks. You can use the dcolen attribute to specify a different size. If specified, the size of the plex must be a integer multiple of 33 blocks from 33 up to a maximum of 2112 blocks.

You can 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 0 DCO Plexes for details.

Specifying Storage for Version 0 DCO Plexes

If the disks that contain volumes and their snapshots are to be moved or split into different disk groups, the disks that contain their respective DCO plexes must be able to accompany them. By default, VxVM attempts to place the DCO plexes on the same disks as the data plexes of the parent volume. However, this may be impossible if there is insufficient space available on those disks. In this case, VxVM uses any available space on other disks in the disk group. If the DCO plexes are placed on disks which are used to hold the plexes of other volumes, this may cause problems when you subsequently attempt to move volumes into other disk groups.

You can use storage attributes to specify explicitly which disks to use for the DCO plexes. If possible, specify the same disks as those on which the volume is configured. For example, to add a DCO object and DCO volume with plexes on mydg05 and mydg06, and a plex size of 264 blocks to the volume, myvol, in the disk group, mydg, 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 partial vxprint 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) manual page.

Removing a Version 0 DCO and DCO Volume

To dissociate a DCO object, DCO volume and any snap objects from a volume, use the following command:

This completely removes the DCO object, DCO volume and any snap objects. It also has the effect of disabling FastResync for the volume.

Alternatively, you can use the vxdco command to the same effect:

The default name of the DCO object, dco_obj, for a volume is usually formed by appending the string _dco to the name of the parent volume. To find out the name of the associated DCO object, use the vxprint command on the volume.

To dissociate, but not remove, the DCO object, DCO volume and any snap objects from the volume, myvol, in the disk group, mydg, use the following command:

This form of the command dissociates the DCO object from the volume but does not destroy it or the DCO volume. If the -o rm option is specified, the DCO object, DCO volume and its plexes, and any snap objects are also removed.

For more information, see the vxassist(1M) and vxdco(1M) manual pages.

Reattaching a Version 0 DCO and DCO Volume

If the DCO object and DCO volume are not removed by specifying the -o rm option to vxdco, they can be reattached to the parent volume using the following command:

For example, to reattach the DCO object, myvol_dco, to the volume, myvol, use the following command:

For more information, see the vxdco(1M) manual page.