Guten Tag David Steele,
am Montag, 8. Juli 2019 um 14:12 schrieben Sie:
> pg_start_backup() does a checkpoint, but then the database continues
> writing as you copy the files in whatever order you choose. You may
> copy a file that has a partial write or copy some files involved in a
> transaction before it happens and others afterwards -- in fact this is
> normal and expected.
And because that's expected, Postgres can successfully restore from
that, e.g. having used checkpoints before:
> [...]This log exists primarily for crash-safety purposes: if the
> system crashes, the database can be restored to consistency by
> “replaying” the log entries made since the last checkpoint.
https://www.postgresql.org/docs/current/continuous-archiving.html
"since the last checkpoint": Missing WAL-segments mean a loss of data
only. It doesn't mean that formerly "checkpointed" data gets magically
broken, else crash recovery wouldn't work like described in the docs.
> The checkpoint constrains the range of WAL that you need, but that WAL
> is absolutely needed to reconstruct the changes that happened during the
> backup.
Which makes sense if all WAL-archives are simply considered to be
incremental changes based on some former full backup. But that's the
point: I don't see how WAL-archives created between pg_start- and
pg_stop_backup are any different to later ones. Of course one needs
those to not loose data at all, but that doesn't tell anything about
how usable the data directory in itself is already without those.
Postgres seems to have simply defined that they additionally care
about the time when a backup is running. Which is fine of course, but
I still don't see any technical or conceptual limitation of not
following that decision. If I backup some VM using snapshots, I don't
necessarily care about the changes made within the VM during the
backup as well. Those are simply handled by the next backup. But there
are additional products streaming all changes to the VM somewhere, if
one needs that.
OTOH, it's of course good to have two other opinions to mine when my
boss asks if things are OK the way they are. :-)
Mit freundlichen Grüßen,
Thorsten Schöning
--
Thorsten Schöning E-Mail: Thorsten.Schoening@AM-SoFT.de
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