A variant of the third-mirror break-off snapshot type are linked break-off snapshot volumes, which use the
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
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:
Synchronization of the mirror volume is in progress. The
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.
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
snapwait command can be used to wait for the state to become
.When you use the
make command to create the snapshot volume, this removes the link, and establishes a snapshot relationship between the snapshot volume and the original volume.
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 339.
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 331.