Hi Magnus, > On 18. Jun, 2020, at 16:19, Magnus Hagander <mag...@hagander.net> wrote: > I don't know specifically about SnapCenter, but for snapshots in general, it > does require backup mode *unless* all your data is on the same disk and you > have an atomic snapshot across that disk (in theory it can be on different > disk as well, as long as the snapshots in that case are atomic across *all* > those disks, not just individually, but that is unusual).
according to what I know from our storage guys, Netapp does atomic snapshots for each volume. We have the database and its corresponding WAL files (pg_wal directory) on the same volume and the archived WALs (archive_command) on another. And the snapshots on those two volumes are not taken at the same time. Currently, the database is set to backup mode (using cron) and the storage guys have a window during which they can take the snapshots. > So the upthread suggestion of putting data and wal on different disk and > snapshoting them at different times is *NOT* safe. Unless the reference to > the directory for the logs means a directory where log files are copied out > with archive_command, and it's actually the log archive (in which case it > will work, but the recommendation is that the log archive should not be on > the same machine). as I said above, pg_wal is a directory in PGDATA at the default location and WALs are archived using the archive_command to a different volume. So I guess, we should be safe then. > The normal case is that snapshots are guaranteed to be atomic, and thus this > millisecond window cannot appear. But that usually only applies across > individual volumes. It's also worth noticing that taking the backups without > using the backup mode and archive_command means you cannot use them for PITR, > only for restore onto that specific snapshot. > > While our documentation on backups in general definitely needs improvement, > this particular requirement is documented at > https://www.postgresql.org/docs/current/backup-file.html. since we use backup mode, we should be good with PITR too. I didn't test that myself, but a workmate did and he said it worked nicely. So bottom line, SnapCenter means for PostgreSQL just a plain volume snapshot like we currently do with SnapCreator, using backup mode scripts and "archive_command"-ing WALs to a diffent volume, i.e. no change in strategy. It's just that SnapCenter can't control backup mode as it can with Oracle. Thanks for the invaluable input. Also, thanks Ken. Cheers, Paul
