Skip site navigation (1) Skip section navigation (2)

Re: Recovery conflict monitoring

From: Magnus Hagander <magnus(at)hagander(dot)net>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Recovery conflict monitoring
Date: 2011-01-03 09:03:36
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Mon, Jan 3, 2011 at 00:23, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> 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.

Thanks for reviewing!

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

They shouldn't - not more than other stats messages. Those are often
flushed from on_shmem_exit() which I think runs even later.

> * 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.

Depends on the usage scenario. In a lot of dedicated environments you
really only have one database - but there are many environments where
you do have multiple and it's quite useful to see them separately. And
you can of course very easily sum() them up for a total count, since
it's a view... Better keep the detail than throw it away, even if that
part isn't useful in *all* cases...

Grouping by user would potentially be helpful - I agree. However, that
goes for most pgstat counters ("number of seqscans", "tuples read" etc
are interesting per user as well in some cases). So doing that right
would mean adding per-user tracking all across pgstats in some smart
way - something we don't do now at all. So I see that as a separate

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

Logging is useful to figure out why you have a certain scenario, yes.
But absolutely not as a *replacement* for the statistics counters, but
as an addition. Just like we have (the now incorrectly named)
pg_stat_bgwriter view *and* log_checkpoints... Different usecases for
the same basic information.

> 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.

It will help reduce it, but not take it away, right? Plus, it's an
optional feature...

 Magnus Hagander

In response to


pgsql-hackers by date

Next:From: Joel JacobsonDate: 2011-01-03 09:15:34
Subject: Re: contrib/snapshot
Previous:From: Jim NasbyDate: 2011-01-03 09:02:25
Subject: Re: page compression

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group