Re: Incremental backup from a streaming replication standby fails

On Wed, 2024-07-24 at 15:27 -0400, Robert Haas wrote:
> On Wed, Jul 24, 2024 at 6:46 AM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote:
> >    An incremental backup is only possible if replay would begin from a later
> >    checkpoint than the checkpoint that started the previous backup upon which
> >    it depends.
>
> My concern here is that the previous backup might have been taken on a
> standby, and therefore it did not start with a checkpoint. For a
> standby backup, replay will begin from a checkpoint record, but that
> record may be quite a bit earlier in the WAL. For instance, imagine
> checkpoint_timeout is set to 30 minutes on the standby. When the
> backup is taken, the most recent restartpoint could be up to 30
> minutes ago -- and it is the checkpoint record for that restartpoint
> from which replay will begin. I think that in my phrasing, it's always
> about the checkpoint from which replay would begin (which is always
> well-defined) not the checkpoint that started the backup (which is
> only logical on the primary).

I see.

The attached patch uses your wording for the first sentence.

I left out the last sentence from your suggestion, because it sounded
like it is likely to confuse the reader. I think you just wanted to
say that there are other possible causes for an incremental backup to
fail. I want to keep the text as simple as possible and focus on the case
that I hit, because I expect that a lot of people who experiment with
incremental backup or run tests could run into the same problem.

I don't think it will be a frequent occurrence during normal operation.

Yours,
Laurenz Albe