Date: Tue, 04 Apr 2006 17:51:26 -0400 From: Mike Jakubik <mikej@rogers.com> To: Eric Anderson <anderson@centtech.com> Cc: stable@freebsd.org, pjd@freebsd.org, Craig Boston <craig@feniz.gank.org> Subject: Re: gmirror on existing filesystem (was Fresh install on gmirror'ed disks?) Message-ID: <4432EA5E.3060003@rogers.com> In-Reply-To: <442043D0.6090206@centtech.com> References: <440D74B3.3030309@vwsoft.com> <200603070939.30032.joao@matik.com.br> <54559.192.168.0.10.1141751042.squirrel@webmail.sd73.bc.ca> <20060316160813.GA15720@nowhere> <442033A2.2030208@rogers.com> <442043D0.6090206@centtech.com>
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Eric Anderson wrote: > Mike Jakubik wrote: >> 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? > > > I've gmirrored existing disks/slices before, and it's worked fine. > I'm not 100% certain about all cases, but it's possible that the > filesystem could be right up against the last block of the partition, > and it could get stomped on I suppose. > > I'm not sure what this command tells you for sure, but it dumps the > last block of a slice, or disk, or whatever: > > > dd if=/dev/ad0s3a iseek=`diskinfo ad0s3a | perl -ne '@d = split; print > ($d[2]/$d[1] - 1)'` count=512 | hexdump Could someone provide an authoritative answer to this please? Pawel, it would be nice to see some support for your own code from you. This is a very easy method to create a mirror on an existing system, but if its going to cause problems then its useless (All the more reason for geom enabled installer).home | help
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