gmirror on existing filesystem (was Fresh install on gmirror'ed disks?)

[Craig Boston]

On Tue, Mar 07, 2006 at 09:04:02AM -0800, Freddie Cash wrote:
> There's no need to copy files around.  gmirror handles it all for you
> behind the scenes.  Just create the gmirror labels using the existing
> disks/slices/partitions, then insert the second set of
> disks/slices/parittions.  gmirror will handle synchonising the data
> across the mirror.

AFAIK, gmirror causes whatever provider it's mirroring to "lose" the
last block to metadata.  I've always avoided mirroring an existing
filesystem for fear that shrinking a UFS filesystem's underlying device
might cause problems down the road.

Can someone with knowledge of the UFS internals please confirm one way
or the other if this is dangerous or not?

Craig