Re: pg15b3: recovery fails with wal prefetch enabled

On Mon, Sep 5, 2022 at 1:28 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> I had this more or less figured out on Friday when I wrote last, but I
> got stuck on a weird problem with 026_overwrite_contrecord.pl. I
> think that failure case should report an error, no? I find it strange
> that we end recovery in silence. That was a problem for the new
> coding in this patch, because it is confused by XLREAD_FAIL without
> queuing an error, and then retries, which clobbers the aborted recptr
> state. I'm still looking into that.

On reflection, it'd be better not to clobber any pre-existing error
there, but report one only if there isn't one already queued. I've
done that in this version, which I'm planning to do a bit more testing
on and commit soonish if there are no comments/objections, especially
for that part.

I'll have to check whether a doc change is necessary somewhere to
advertise that maintenance_io_concurrency=0 turns off prefetching, but
IIRC that's kinda already implied.

I've tested quite a lot of scenarios including make check-world with
maintenance_io_concurrency = 0, 1, 10, 1000, and ALTER SYSTEM for all
relevant GUCs on a standby running large pgbench to check expected
effect on pg_stat_recovery_prefetch view and generate system calls.