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Re: Streaming replication, and walsender during recovery

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication, and walsender during recovery
Date: 2010-01-28 19:13:18
Message-ID: 1264705998.24669.11552.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, 2010-01-28 at 21:00 +0200, Heikki Linnakangas wrote:
> However, since not every checkpoint is a restartpoint we might easily
> > end up with significantly more WAL files on the standby than would
> > normally be there when it would be a primary. Not sure if that is an
> > issue in this case, but we can't just assume we can store all files
> > needed to restart the standby on the standby itself, in all cases.
> That
> > might be an argument to add a restartpoint_segments parameter, so we
> can
> > trigger restartpoints on WAL volume as well as time. But even that
> would
> > not put an absolute limit on the number of WAL files.
> 
> I think it is a pretty important safety feature that we keep all the
> WAL around that's needed to recover the standby. To avoid
> out-of-disk-space situation, it's probably enough in practice to set
> checkpoint_timeout small enough in the standby to trigger
> restartpoints often enough.

Hmm, I'm sorry but that's bogus. Retaining so much WAL that we are
strongly in danger of blowing disk space is not what I would call a
safety feature. Since there is no way to control or restrain the number
of files for certain, that approach seems fatally flawed. Reducing
checkpoint_timeout is the opposite of what you would want to do for
performance.

-- 
 Simon Riggs           www.2ndQuadrant.com


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