Re: Track IO times in pg_stat_io

> >>> Now I've a second thought: what do you think about resetting the related number
> >>> of operations and *_time fields when enabling/disabling track_io_timing? (And mention it in the doc).
> >>>
> >>> That way it'd prevent bad interpretation (at least as far the time per operation metrics are concerned).
> >>>
> >>> Thinking that way as we'd loose some (most?) benefits of the new *_time columns
> >>> if one can't "trust" their related operations and/or one is not sampling pg_stat_io frequently enough (to discard the samples
> >>> where the track_io_timing changes occur).
> >>>
> >>> But well, resetting the operations could also lead to bad interpretation about the operations...
> >>>
> >>> Not sure about which approach I like the most yet, what do you think?
> >>
> >> Oh, this is an interesting idea. I think you are right about the
> >> synchronization issues making the statistics untrustworthy and, thus,
> >> unuseable.
> > 
> > No, I don't think we can do that. It can be enabled on a per-session basis.

> Oh right. So it's even less clear to me to get how one would make use of those new *_time fields, given that:

> - pg_stat_io is "global" across all sessions. So, even if one session is doing some "testing" and needs to turn track_io_timing on, then it
> is even not sure it's only reflecting its own testing (as other sessions may have turned it on too).

> - There is the risk mentioned above of bad interpretations for the "time per operation" metrics.

> - Even if there is frequent enough sampling of it pg_stat_io, one does not know which samples contain track_io_timing changes (at the cluster or session level).

As long as track_io_timing can be toggled, blk_write_time could lead to wrong conclusions.
I think it may be helpful to track the blks_read when track_io_timing is enabled
Separately.

blks_read will be as is and give the overall blks_read, while a new column
blks_read_with_timing will only report on blks_read with track_io_timing enabled.

blks_read_with_timing should never be larger than blks_read.

This will then make the blks_read_time valuable if it's looked at with
the blks_read_with_timing column.


Regards,

-- 

Sami Imseih
Amazon Web Services (AWS)