Subject: Re: [PATCHES] Infrastructure changes for recovery
From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: List pgsql-patches <pgsql-patches@postgresql.org>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
In-Reply-To: <12423.1222701848@sss.pgh.pa.us>
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Date: Mon, 29 Sep 2008 16:54:55 +0100
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On Mon, 2008-09-29 at 11:24 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > On Mon, 2008-09-29 at 10:13 -0400, Tom Lane wrote:
> >> ... If we crash and restart, we'll have to get to the end
> >> of this file before we start letting backends in; which might be further
> >> than we actually got before the crash, but not too much further because
> >> we already know the whole WAL file is available.
> 
> > Don't want to make it per file though. Big systems can whizz through WAL
> > files very quickly, so we either make it a big number e.g. 255 files per
> > xlogid, or we make it settable (and recorded in pg_control).
> 
> I think you are missing the point I made above.  If you set the
> okay-to-resume point N files ahead, and then the master stops generating
> files so quickly, you've got a problem --- it might be a long time until
> the slave starts letting backends in after a crash/restart.
> 
> Fetching a new WAL segment from the archive is expensive enough that an
> additional write/fsync per cycle doesn't seem that big a problem to me.
> There's almost certainly a few fsync-equivalents going on in the
> filesystem to create and delete the retrieved segment files.

Didn't miss yer point, just didn't agree. :-)

I'll put it at one (1) and then wait for any negative perf reports. No
need to worry about things like that until later.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support