On Sat, 2020-06-13 at 19:48 +0200, Peter wrote: > ! > 4. If, by misconfiguration and/or operator error, the backup system > ! > happens to start a second backup. in parallel to the first, > ! > then do I correctly assume, both backups will be rendered > ! > inconsistent while this may not be visible to the operator; and > ! > the earlier backup would be flagged as apparently successful while > ! > carrying the wrong (later) label? > ! > ! If you are using my scripts and start a second backup while the first > ! one is still running, the first backup will be interrupted. > > This is not what I am asking. It appears correct to me, that, on > the database, the first backup will be interrupted. But on the > tape side, this might go unnoticed, and on completion it will > successfully receive the termination code from the *SECOND* > backup - which means that on tape we will have a seemingly > successful backup, which > 1. is corrupted, and > 2. carries a wrong label. That will only happen if the backup that uses my scripts does the wrong thing. An example: - Backup #1 calls "pgpre.sh" - Backup #1 starts copying files - Backup #2 calls "pgpre.sh". This will cancel the first backup. - Backup #1 completes copying files. - Backup #1 calls "pgpost.sh". It will receive an error. So it has to invalidate the backup. - Backup #2 completes copying files. - Backup #2 calls "pgpost.sh". It gets a "backup_label" file and completes the backup. So the only way that something can go wrong would be if backup #1 somehow does *not* invalidate the backup. > ! This is specific to my scripts, PostgreSQL's non-exclusive backup > ! can perform more than one concurrent backup successfully. > ! I tried to keep things simple. > > I understand. But the operator may not know that and/or accidentially > start a second backup while one is still running. And this will then > result in ... > > ! If you have the wrong "backup_label", you end up with silent data corruption. > > ... this. Of course, if you do arbitrary nonsense like restoring a backup without "backup_label", you will get arbitrary data corruption. It is a fundamental principle that, apart from "backup_label", there is no way to tell a backup from a crashed data directory. Any backup/recovery software must refuse to use a backup that lacks that file. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
