Re: What's the dfifference between pg_start_backup+copy+pg_stop_backup+WAL vs. pg_start_backup+pg_stop_backup+copy+WAL?

Guten Tag Stephen Frost,
am Freitag, 23. Juli 2021 um 17:24 schrieben Sie:

> [...]Similar, once you reach the BACKUP_END record of
> a backup you know the database is consistent- but only if all of the
> files that PG was writing to were copied between the start checkpoint
> of the backup and the BACKUP_END record in the WAL.

So I am correct that the recovery process starts at what is documented
as CHECKPOINT LOCATION in the file backup_label? If the data directory
looks like a more current checkpoint might be available, that is NOT
used? Instead, all available WALs are replayed?

But how come the state of other data files into the recovery process?
that's what Laurenz Albe's example is about: A data file which is more
current than expected because that file is only copied, but the
associated changes are stored in WALs after pg_stop_backup and
therefore need to be considered lost.

Why is the data file with that additional row in B used at all? The
additional row is associated with a transaction number not available
when restoring based on CHECKPOINT LOCATION + WALs of pg_stop_backup.

So why is that row used at all instead of simply ignored? That would
actually revert the cluster to the state of pg_stop_backup, but as
said, backup outdate anyway. I still don't get why the result is not
as consistent as e.g. a crash at the same point in time instead of
doing a backup.

Mit freundlichen Grüßen

Thorsten Schöning

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Für Rückfragen stehe ich Ihnen sehr gerne zur Verfügung.

Mit freundlichen Grüßen

Thorsten Schöning


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