Date: Fri, 4 Jun 2021 17:52:17 -0400
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	Andres Freund <andres@anarazel.de>,
	Magnus Hagander <magnus@hagander.net>,
	Pg Hackers <pgsql-hackers@postgresql.org>, lukas@fittl.com,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and
 more?)
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On 2021-Apr-12, Melanie Plageman wrote:

> As for the way I have recorded strategy writes -- it is quite inelegant,
> but, I wanted to make sure that I only counted a strategy write as one
> in which the backend wrote out the dirty buffer from its strategy ring
> but did not check if there was any clean buffer in shared buffers more
> generally (so, it is *potentially* an avoidable write). I'm not sure if
> this distinction is useful to anyone. I haven't done enough with
> BufferAccessStrategies to know what I'd want to know about them when
> developing or using Postgres. However, if I don't need to be so careful,
> it will make the code much simpler (though, I'm sure I can improve the
> code regardless).

I was bitten last year by REFRESH MATERIALIZED VIEW counting its writes
via buffers_backend, and I was very surprised/confused about it.  So it
seems definitely worthwhile to count writes via strategy separately.
For a DBA tuning the server configuration it is very useful.

The main thing is to *not* let these writes end up regular
buffers_backend (or whatever you call these now).  I didn't read your
patch, but the way you have described it seems okay to me.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W