Re: wait events for disk I/O

On Tue, Mar 7, 2017 at 9:32 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Tue, Mar 7, 2017 at 9:16 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Mon, Mar 6, 2017 at 9:09 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> 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.
>>
>> I think writes have a chance of blocking in cases even in cases that
>> are not very obscure at all.
>
> Point taken for writes, but I think in general we should have some
> criteria based on which we can decide whether to have a wait event for
> a particular call. It should not happen that we have tons of wait
> events and out of which, only a few are helpful in most of the cases
> in real-world scenarios.

Well, the problem is that if you pick and choose which wait events to
add based on what you think will be common, you're actually kind of
hosing yourself. Because now when something uncommon happens, suddenly
you don't get any wait event data and you can't tell what's happening.
I think the number of new wait events added by Rushabh's patch is
wholly reasonable. Yeah, some of those are going to be a lot more
common than others, but so what? We add wait events so that we can
find out what's going on. I don't want to sometimes know when a
backend is blocked on an I/O. I want to ALWAYS know.

--
Robert Haas
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