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

[Mike Jakubik]

Craig Boston wrote:
> 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?
>   

I'm curious to know this as well, as i have some systems using gmirror, 
that were setup in this fashion. Could someone knowledgeable on the 
matter shed some light?