[linux-lvm] Keep snapshots active for 24 hours?

Steve Wray steve.wray at the.net.nz
Fri Dec 14 15:50:02 UTC 2001

I'd been wondering if snapshotting could be used for;

- providing 'virtual journalling' any filesystem that happens to
be on a logical volume.

- take a snapshot, install something or try something out
that might break something, then restore *directly* from
the snapshot if anything goes wrong; without having to actually
back it up to media and restore it.

> -----Original Message-----
> From: linux-lvm-admin at sistina.com [mailto:linux-lvm-admin at sistina.com]On
> Behalf Of Andreas Dilger
> Sent: Saturday, 15 December 2001 07:38
> To: Dave Alden
> Cc: linux-lvm at sistina.com
> Subject: Re: [linux-lvm] Keep snapshots active for 24 hours?
> On Dec 14, 2001  07:35 -0500, Dave Alden wrote:
> >   I'm in the process of evaluating LVM (with XFS) for our ~500G RAID NFS
> > server.  I'm using hardware RAID, so I'm only using LVM is for
> snapshots.
> > My original plan was to take a snapshot around midnight
> everyday, run the
> > backup, then remove the snapshot (just as the FAQ says to :-).  However,
> > I'd like to offer my users the ability to have access to the
> previous days
> > snapshot (so they can easily restore an accidentally deleted
> file).  I don't
> > need all 500G (we're currently using only ~60G), so I was
> thinking of either
> > 50G or 100G for the snapshot (although I only expect ~5G/day to
> be changed).
> > I was wondering what kind of performance hit would I take if I kept the
> > snapshot for 24 hours?
> Well, it will basically mean [read-]write-write for all data going to the
> disk.  Generally, this will not matter a huge amount unless you are doing
> tons of I/O.  You could start with a large snapshot size, and then get an
> idea of how much data really changes each day.  You could then actually
> keep multiple snapshots if you had the space.  If you were really clever,
> you could write a script which checked the free space in the snapshots
> every minute, and if they had less than X PEs free, you extend
> the snapshot
> LV to have more free space.  If you run out of free PEs, you delete the
> oldest snapshot.
> Cheers, Andreas
> --
> Andreas Dilger
> http://sourceforge.net/projects/ext2resize/
> http://www-mddsp.enel.ucalgary.ca/People/adilger/
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