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

70 Understanding Veritas Volume Manager
FastResync
In either case, the part of the map that corresponds to the grown area of the volume is
marked as “dirty” so that this area is resynchronized. The
snapback operation fails if it
attempts to create an incomplete snapshot plex. In such cases, you must grow the replica
volume, or the original volume, before invoking any of the commands
vxsnap reattach,
vxsnap restore, or vxassist snapback. Growing the two volumes separately can
lead to a snapshot that shares physical disks with another mirror in the volume. To prevent
this, grow the volume after the
snapback command is complete.
FastResync limitations
The following limitations apply to FastResync:
Persistent FastResync is supported for RAID-5 volumes, but this prevents the use of
the relayout or resize operations on the volume while a DCO is associated with it.
Neither non-persistent nor persistent FastResync can be used to resynchronize
mirrors after a system crash. Dirty region logging (DRL), which can coexist with
FastResync, should be used for this purpose. In VxVM 4.0 and later releases, DRL
logs may be stored in a version 20 DCO volume.
When a subdisk is relocated, the entire plex is marked “dirty” and a full
resynchronization becomes necessary.
If a snapshot volume is split off into another disk group, non-persistent FastResync
cannot be used to resynchronize the snapshot plexes with the original volume when
the disk group is rejoined with the original volume’s disk group. Persistent
FastResync must be used for this purpose.
If you move or split an original volume (on which persistent FastResync is enabled)
into another disk group, and then move or join it to a snapshot volume’s disk group,
you cannot use
vxassist snapback to resynchronize traditional snapshot plexes
with the original volume. This restriction arises because a snapshot volume
references the original volume by its record ID at the time that the snapshot volume
was created. Moving the original volume to a different disk group changes the
volume’s record ID, and so breaks the association. However, in such a case, you can
use the
vxplex snapback command with the -f (force) option to perform the
snapback.
Note: This restriction only applies to traditional snapshots. It does not apply to instant
snapshots.
Any operation that changes the layout of a replica volume can mark the FastResync
change map for that snapshot “dirty” and require a full resynchronization during
snapback. Operations that cause this include subdisk split, subdisk move, and online
relayout of the replica. It is safe to perform these operations after the snapshot is