Re: AIO v2.5

On Tue, Mar 25, 2025 at 11:57:58AM -0400, Andres Freund wrote:
> On 2025-03-25 07:11:20 -0700, Noah Misch wrote:
> > On Mon, Mar 24, 2025 at 10:52:19PM -0400, Andres Freund wrote:
> > > If we want to implement it, I think we could introduce PGAIO_RS_WARN, which
> > > then could tell the stager to issue the WARNING. It would add a bit of
> > > distributed cost, both to callbacks and users of AIO, but it might not be too
> > > bad.
>
> FWIW, I prototyped this, it's not hard.
>
> But it can't replace the current WARNING with 100% fidelity: If we read 60
> blocks in a single smgrreadv, we today would would emit 60 WARNINGs. But we
> can't encode that many block offset in single PgAioResult, there's not enough
> space, and enlarging it far enough doesn't seem to make sense either.
>
>
> What we *could* do is to emit one WARNING for each bufmgr.c smgrstartreadv(),
> with that warning saying that there were N zeroed blocks in a read from block
> N to block Y and a HINT saying that there are more details in the server log.

Sounds fine.

> > Another thought on complete_shared running on other backends: I wonder if we
> > should push an ErrorContextCallback that adds "CONTEXT: completing I/O of
> > other process" or similar, so people wonder less about how "SELECT FROM a" led
> > to a log message about IO on table "b".
>
> I've been wondering about that as well, and yes, we probably should.
>
> I'd add the pid of the backend that started the IO to the message - although
> need to check whether we're trying to keep PIDs of other processes from
> unprivileged users.

We don't.

> I think we probably should add a similar, but not equivalent, context in io
> workers. Maybe "I/O worker executing I/O on behalf of process %d".

Sounds good.