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. > 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. It's simply: 1. create a snapshot (with a note that LVM1 defaults to read-only, while LVM2 defaults to read/write. I suggest using read-only snapshots) 2. mount the snapshot 3. do the backup 4. unmount and lvremove the snapshot ... 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. Hope this helps! - Graham
