Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

303Administering volume snapshots
Linked break-off snapshot volumes
For information about how to add snapshot mirrors to a volume, see “Adding snapshot
mirrors to a volume” on page 325.
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:
ACTIVE The mirror volume has been fully synchronized from the original
volume. The
vxsnap make command can be run to create a snapshot
volume.
ATTACHING 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.
BROKEN 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 263.
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