Re: New statistics for WAL buffer dirty writes

(2012/12/10 3:06), Tomas Vondra wrote:
> On 29.10.2012 04:58, Satoshi Nagayasu wrote:
>> 2012/10/24 1:12, Alvaro Herrera wrote:
>>> Satoshi Nagayasu escribi�:
>>>
>>>> With this patch, walwriter process and each backend process
>>>> would sum up dirty writes, and send it to the stat collector.
>>>> So, the value could be saved in the stat file, and could be
>>>> kept on restarting.
>>>>
>>>> The statistics could be retreive with using
>>>> pg_stat_get_xlog_dirty_writes() function, and could be reset
>>>> with calling pg_stat_reset_shared('walwriter').
>>>>
>>>> Now, I have one concern.
>>>>
>>>> The reset time could be captured in globalStats.stat_reset_timestamp,
>>>> but this value is the same with the bgwriter one.
>>>>
>>>> So, once pg_stat_reset_shared('walwriter') is called,
>>>> stats_reset column in pg_stat_bgwriter does represent
>>>> the reset time for walwriter, not for bgwriter.
>>>>
>>>> How should we handle this?  Should we split this value?
>>>> And should we have new system view for walwriter?
>>>
>>> I think the answer to the two last questions is yes.  It doesn't seem to
>>> make sense, to me, to have a single reset timings for what are
>>> effectively two separate things.
>>>
>>> Please submit an updated patch to next CF.  I'm marking this one
>>> returned with feedback.  Thanks.
>>>
>>
>> I attached the latest one, which splits the reset_time
>> for bgwriter and walwriter, and provides new system view,
>> called pg_stat_walwriter, to show the dirty write counter
>> and the reset time.
>
> I've done a quick review of the v4 patch:

Thanks for the review, and sorry for my delayed response.

> 1) applies fine on HEAD, compiles fine
>
> 2) "make installcheck" fails because of a difference in the 'rules'
>     test suite (there's a new view "pg_stat_walwriter" - see the
>     attached patch for a fixed version or expected/rules.out)

Ah, I forgot about the regression test. I will fix it. Thanks.

> 3) I do agree with Alvaro that using the same struct for two separate
>     components (bgwriter and walwriter) seems a bit awkward. For example
>     you need to have two separate stat_reset fields, the reset code
>     becomes much more verbose (because you need to list individual
>     fields) etc.
>
>     So I'd vote to either split this into two structures or keeping it
>     as a single structure (although with two views on top of it).

Ok, I will split it into two structs, PgStat_BgWriterGlobalStats and 
PgStat_WalWriterGlobalStats, and will modify PgStat_GlobalStats to hold 
those two structs in the stat collector.

> 4) Are there any other fields that might be interesting? Right now
>     there's just "dirty_writes" but I guess there are other values. E.g.
>     how much data was actually written etc.?

AFAIK, I think those numbers can be obtained by calling 
pg_current_xlog_insert_location() or pg_current_xlog_location(),
but if we need it, I will add it.

Regards,

>
> Tomas
>
>
>
>


-- 
Satoshi Nagayasu <snaga@uptime.jp>
Uptime Technologies, LLC. http://www.uptime.jp