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Re: Recovery conflict monitoring

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Magnus Hagander <magnus(at)hagander(dot)net>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Recovery conflict monitoring
Date: 2011-01-02 23:23:19
Message-ID: 1294010599.2090.4121.camel@ebony (view raw or whole thread)
Lists: pgsql-hackers
On Mon, 2010-12-27 at 14:39 +0100, Magnus Hagander wrote:
> On Thu, Dec 23, 2010 at 13:09, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> > This patch adds counters and views to monitor hot standby generated
> > recovery conflicts. It extends the pg_stat_database view with one
> > column with the total number of conflicts, and also creates a new view
> > pg_stat_database_conflicts that contains a breakdown of exactly what
> > caused the conflicts.
> >
> > Documentation still pending, but comments meanwhile is of course appreciated ;)
> Heikki pointed out over IM that it's pointless to count stats caused
> by recovery conflict with drop database - since we drop the stats
> record as soon as it arrives anyway. Here's an updated patch that
> removes that, and also adds some documentation.

I like the patch, well inspired, code in the right places AFAICS. No
code comments at all.

Couple of thoughts: 

* are we safe to issue stats immediately before issuing FATAL? Won't
some of them get lost?

* Not clear what I'd do with database level information, except worry a
lot. Maybe an option to count conflicts per user would be better, since
at least we'd know exactly who was affected by those. Just an idea.

* Would it better to have a log_standby_conflicts that allowed the
opportunity to log the conflicting SQL, duration until cancelation etc?

I'd rather have what you have than nothing at all though... the new
hot_standby_feedback mode should be acting to reduce these, so it would
be useful to have this patch enabled for testing that feature.

 Simon Riggs 
 PostgreSQL Development, 24x7 Support, Training and Services

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