Date: Thu, 06 Apr 2006 14:56:20 -0500 From: Eric Anderson <anderson@centtech.com> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: Mike Jakubik <mikej@rogers.com>, stable@FreeBSD.org, Craig Boston <craig@feniz.gank.org> Subject: Re: gmirror on existing filesystem (was Fresh install on gmirror'ed disks?) Message-ID: <44357264.6000600@centtech.com> In-Reply-To: <20060406081213.GA5578@garage.freebsd.pl> 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> <4432EA5E.3060003@rogers.com> <20060406081213.GA5578@garage.freebsd.pl>
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Pawel Jakub Dawidek wrote: > On Tue, Apr 04, 2006 at 05:51:26PM -0400, Mike Jakubik wrote: > +> >>>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). > > I can't give you an authoritative answer, because I don't know UFS > internals so well. All I know is that it (UFS) thinks the last sector is > available and may want to use it at some point getting EIO then. > I'm not using this method, but I've heard of many people using it > without problems. > > Speaking about installer. I don't think I'll be able to add > configuration of my GEOM classes to the sysinstall in the near future > (and I hope never - I'd prefer to wait for a new installer). > One can still see how many sectors exactly has the partition he is going > to create file system on and add additional newfs(8) flag > '-s <sectors-1>'. > I suppose one could look at the output of diskinfo, and compare against the output of df, and see if there is a spare 512+ bytes at the end of the partition. I think there's a possibility that newfs won't use the last chunk if it's less than BLOCKSIZE bytes.. Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
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