Re: wait events for disk I/O

On Tue, Mar 7, 2017 at 5:27 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> 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.
>

Sure, if you think both Writes and Reads at OS level can have some
chance of blocking in obscure cases, then we should add a wait event
for them.

> 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.
>

Yeah, that makes sense to me and we should try for both read-write and
read-only tests.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com