Date: Fri, 1 Aug 2008 07:31:27 -0600 From: Modulok <modulok@gmail.com> To: "Matthew Seaman" <m.seaman@infracaninophile.co.uk> Cc: freebsd-questions@freebsd.org Subject: Re: dumping mounted file systems with insufficient space... Message-ID: <64c038660808010631x44ec934cnc04dd8ab17d49519@mail.gmail.com> In-Reply-To: <48903C4C.2090902@infracaninophile.co.uk> References: <64c038660807300154ue4e5a0cyb792b46fd3ec6037@mail.gmail.com> <48903C4C.2090902@infracaninophile.co.uk>
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>> I can use dump(8) an active, mounted file systems via the -L flag. >> According to the manual, this first creates a snapshot of the file >> system, to the .snap directory of the file systems root. What if the >> file system to be dumped, does not have sufficient free-space to store >> a snapshot? Can I still safely dump(8) a mounted file system? > > A snapshot doesn't take any significant /extra/ space itself. Rather it > consists of marking the state of the system at that time and provides a > view (via the .snap directory) of that state of the filesystem. Of course, > subsequent modifications of the filesystem can cause more space than > otherwise expected to be used up -- as both the snapshot and the latest > versions of anything have to be kept around -- but how much impact this has > depends entirely on the IO traffic characteristics of your particular > filesystem and cannot be predicted in any useful fashion without a great > deal more information. > > If snapshots won't work for you, another trick (if you can swing it) is to > have the data on a RAID1 mirror. Then you can detach one of the mirrors, > back it up and then reattach the mirror. Doing this with gmirror is a > simple matter of writing about a 10 line shell script. Other mirroring > hard/soft-ware may be less cooperative. However you do it, this will > involve an extended period while mirrors resynchronise after the backup > where your file system won't have the desired level of resilience. > > If you can't use snapshots, can't split the mirror and you can't unmount > the filesystem, then the next best thing is to make the filesystem as > quiescent as possible. Basically, shut down any processes using the > filesystem. That's probably as unacceptable as any of the other > alternatives -- in which case, you can still go ahead and dump the > filesystem, but don't expect the generated dump to be 100% consistent. > It will be 'good enough' for some purposes, but files actively involved > in IO at the time the dump is made are likely to be corrupted. > > Cheers, > > Matthew > > -- > Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard > Flat 3 > PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate > Kent, CT11 9PW Thank you for the clarification about snapshots, Matthew. I went with a dump to another disk and it worked out without any problems :) -Modulok-
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