Re: wait events for disk I/O

On Mon, Mar 6, 2017 at 3:27 AM, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com> wrote:
> Yes, I thought of adding wait event only for the sync but then recording the
> wait event for both write and sync. I understand that OS level writes are
> cheap but it still have some cost attached to that. Also I thought for the
> monitoring tool being develop using this wait events, will have more useful
> capture data if we try to collect as much info as we can. Or may be not.
>
> I am open for other opinion/suggestions.

Writes are NOT always fast. I've seen cases of write() blocking for
LONG periods of time on systems that are in the process of failing, or
just busy. So I think we certainly want to advertise a wait event for
those.

Likewise, I agree that the prefetch call probably SHOULDN'T block, but
just because it shouldn't doesn't mean it never will.

I think somebody should try a pgbench run with this patch applied,
using a scale factor greater than shared_buffers, and generate a wait
event profile, just to see if these are showing up and how often.

--
Robert Haas
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