On 7/23/21 11:24 AM, Stephen Frost wrote:
* Thorsten Schöning (tschoening@xxxxxxxxxx) wrote:
Guten Tag Laurenz Albe,
am Freitag, 23. Juli 2021 um 12:41 schrieben Sie:
That recovering is the only difference: In case of file system
snapshots it recovers beginning from the last CHECKPOINT, as that is
considered safe, and applies additional WALs as available.
Yes, it's understood that everything written to the heap prior to the
latest checkpoint was sync'd, and trustworthy, and everything written
after that point is in the WAL, and we know the end of that WAL because
we read through the WAL that existed at the time and discover what the
end point was.
There is another wrinkle here. If you copy files outside of
pg_start/stop_backup() then postgres will not replay over any torn (and
in some cases zero) pages, even if a full page write is available. Torn
pages are expected during crash recovery and backup recovery before the
end-of-backup WAL record. Outside of those cases you'll get a fatal error.
In that case your backup would be entirely useless, even if you knew
exactly where to replay to.
Regards,
--
-David
david@xxxxxxxxxxxxx
