Re: Filesystem snapshots

[Florian Ermisch <florian.ermisch@alumni.tu-berlin.de>]

Seems like I haven't chimed in yet…

Am 22. April 2016 18:24:01 MESZ, schrieb Graham Percival <gperciva@tarsnap.com>:
> On Sat, Apr 16, 2016 at 10:42:40AM +0100, John wrote:
> > In order to archive data when constant updating takes place, even
> > just an email INBOX or a SQL database, the data has to stop
> > changing. The frozen unchanging data is the source for the archive
> > and an overspill area, called - very misleadingly - a snapshot, can
> > accumulate written blocks of fresh data. When the archive is
> > finished, the snapshot can be merged back into the original files
> > and deleted.
> 
> I've never used LVM, but this sounds suspicious to me.  Shouldn't you
> be
> making the backup from a read-only copy of your filesystem?  I don't
> understand why there is anything that needs to be merged.
> 

Let me point out that LVM takes
snapshots on the block level, not in 
the filesystem. Even NTFS' shadow 
copies are a function of the filesystem
but LVM's snapshots are not, the
filesystem sits on top of the logical 
volume. 

> > My X system has remained up but logged off throughout this nightly
> > crontab root script, and that's kept files open and updating on
> > /home. The merge has only been queued by the lvconvert command and
> > will only take effect when /home is unmounted and then mounted
> > again. I need to automate closing X, unmounting and remounting /home
> > and starting X again and I can't think how to do that neatly within
> > the crontab root script.
> 
> Hmm.  This page about LVM:
> http://www.tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html
> doesn't include any merging. 
> […]
>
> ... thinking through this a bit more, are 
> you storing your tarsnap cache
> directory on the partition which is being 
> backed up?  That would be a
> plausible reason for wanting it to be 
> read/write, but my first thought
> is that it would be to use read-only 
> snapshots and put the cache on 
> another partition.

I agree, have the cache somewhere 
writable, avoid needing to merge and 
take your backup from the read-only 
snapshot.

> Hope this helps!
> - Graham

Regards, Florian