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From: Alvaro Herrera <[email protected]>
To: Melanie Plageman <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Magnus Hagander <[email protected]>
Cc: Pg Hackers <[email protected]>
Cc: [email protected]
Cc: Thomas Munro <[email protected]>
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)
Date: Fri, 4 Jun 2021 17:52:17 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAAKRu_atgf8pnWw4So5Sbi-YhD+Vff-Ac10VEjdvE3By-nM3uA@mail.gmail.com>

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

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