Seems like I haven't chimed in yet… Am 22. April 2016 18:24:01 MESZ, schrieb Graham Percival <[email protected]>: > 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
