A variant of third-mirror break-off snapshots are linked break-off snapshots, which use the vxsnap addmir command to link a specially prepared volume with the data volume. The volume that is used for the snapshot is prepared in the same way as for full-sized instant snapshots. However, unlike full-sized instant snapshots, this volume can be set up in a different disk group from the data volume. This makes linked break-off snapshots especially suitable for recurring off-host processing applications as it avoids the disk group split/join administrative step. As with third-mirror break-off snapshots, you must wait for the contents of the snapshot volume to be synchronized with the data volume before you can use the vxsnap make command to take the snapshot.

When a link is created between a volume and the mirror that will become the snapshot, separate link objects (similar to snap objects) are associated with the volume and with its mirror. The link object for the original volume points to the mirror volume, and the link object for the mirror volume points to the original volume. All I/O is directed to both the original volume and its mirror, and a synchronization of the mirror from the data in the original volume is started.

You can use the vxprint command to display the state of link objects, which appear as type ln. Link objects can have the following states:

If you resize (grow or shrink) a volume, all its ACTIVE linked mirror volumes are also resized at the same time. The volume and its mirrors can be in the same disk group or in different disk groups. If the operation is successful, the volume and its mirrors will have the same size.

If a volume has been grown, a resynchronization of the grown regions in its linked mirror volumes is started, and the links remain in the ATTACHING state until resynchronization is complete. The vxsnap snapwait command can be used to wait for the state to become ACTIVE.

When you use the vxsnap make command to create the snapshot volume, this removes the link, and establishes a snapshot relationship between the snapshot volume and the original volume.

The vxsnap reattach operation re-establishes the link relationship between the two volumes, and starts a resynchronization of the mirror volume.

An empty volume must be prepared for use by linked break-off snapshots.