VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
FastResync
Chapter 1 59
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 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.
• 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 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.
• 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