Re: pg_stat_io not tracking smgrwriteback() is confusing

Hi,

On 2023-04-24 17:37:48 -0400, Melanie Plageman wrote:
> On Mon, Apr 24, 2023 at 02:14:32PM -0700, Andres Freund wrote:
> > It starts blocking once "enough" IO is in flight. For things like an immediate
> > checkpoint, that can happen fairly quickly, unless you have a very fast IO
> > subsystem. So often it'll not matter whether we track smgrwriteback(), but
> > when it matter, it can matter a lot.
>
> I see. So, it sounds like this is most likely to happen for checkpointer
> and not likely to happen for other backends who call
> ScheduleBufferTagForWriteback().

It's more likely, but once the IO subsystem is busy, it'll also happen for
other users ScheduleBufferTagForWriteback().

> Also, it seems like this (given the current code) is only reachable for
> permanent relations (i.e. not for IO object temp relation). If other backend
> types than checkpointer may call smgrwriteback(), we likely have to consider
> the IO context.

I think we should take it into account - it'd e.g. interesting to see a COPY
is bottlenecked on smgrwriteback() rather than just writing the data.

> I would imagine that we want to smgrwriteback() timing to writes/write time
> for the relevant IO context and backend type.

Yes.

Greetings,

Andres Freund