Symantec logo

Linked break-off snapshot volumes

A variant of the third-mirror break-off snapshot type are linked break-off snapshot volumes, 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 off-host processing applications where you may want to create the snapshot on storage with different characteristics from that used for the data volumes. As for 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:


The mirror volume has been fully synchronized from the original volume. The vxsnap make command can be run to create a snapshot volume. 


Synchronization of the mirror volume is in progress. The vxsnap make command cannot be used to create a snapshot volume until the state changes to ACTIVE. The vxsnap snapwait command can be used to wait for the synchronization to complete. 


The mirror volume has been detached from the original volume because of an I/O error or an unsuccessful attempt to grow the mirror volume. The vxrecover command can be used to recover the mirror volume in the same way as for a DISABLED volume. 

See "Starting a volume" on page 280. 

If you resize (that is, 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.

See "Creating and managing linked break-off snapshot volumes" on page 337.

An empty volume must be prepared for use by full-sized instant and linked break-off snapshots.

See "Creating a volume for use as a full-sized instant or linked break-off snapshot" on page 329.