Re: [HACKERS] pg_stat_wal_write statistics view

From: Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Julien Rouhaud <rjuju123(at)gmail(dot)com>, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] pg_stat_wal_write statistics view
Date: 2017-11-27 03:12:26
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On Wed, Nov 8, 2017 at 8:46 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Nov 7, 2017 at 4:31 AM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
> wrote:
> >> Updated patch attached.
> > Patch rebased.
> I think the earlier concerns about the performance impact of this are
> probably very valid concerns, and I don't see how the new version of
> the patch gets us much closer to solving them.

I will check the performance with the changes of removing the stats
usage and provide the details.

> I am also not sure I understand how the backend_write_blocks column is
> intended to work. The only call to pgstat_send_walwrites() is in
> WalWriterMain, so where do the other backends report anything?

With the current patch, All the backends update the stats in shared memory
structure and only WAL writer process gathers the stats and share with the
stats collector.

> Also, if there's only ever one global set of counters (as opposed to
> one per table, say) then why use the stats collector machinery for
> this at all, vs. having a structure in shared memory that can be
> updated directly? It seems like adding a lot of overhead for no
> functional benefit.

Yes, I agree that using stats collector for these stats is an overhead.
I change the patch to use just the shared memory structure and
gather the performance results and post it to the next commitfest.

Currently I marked the patch as "returned with feedback" in the
ongoing commitfest.

Hari Babu
Fujitsu Australia

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