From: Andres Freund <andres@anarazel.de>
To: Florian Weimer <fweimer@bfk.de>
Subject: Re: Wrong docs on checkpoint_segments?
Date: Fri, 7 Jan 2011 14:47:24 +0100
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Cc: pgsql-performance@postgresql.org
References: <82aajcg9wa.fsf@mid.bfk.de>
 <201101071439.32053.andres@anarazel.de>
	<82mxnceskh.fsf@mid.bfk.de>
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On Friday, January 07, 2011 02:45:02 PM Florian Weimer wrote:
> * Andres Freund:
> > On Friday, January 07, 2011 01:45:25 PM Florian Weimer wrote:
> >> On 9.0, this configuration
> >> 
> >> checkpoint_segments = 512     # in logfile segments, min 1, 16MB each
> >> 
> >> results in 1034 segments, so the effective logfile segment size is 32
> >> MB.
> > 
> > Um. Is it possible that you redefined XLOG_SEG_SIZE or used --with-wal-
> > segsize=SEGSIZE?
> 
> No, the individual files are still 16 MB.  It's just that the
> checkpoint_segments limit is not a hard limit, and you end up with
> slightly more than twice the configured number of segments on disk.
Thats documented:
"
There will always be at least one WAL segment file, and will normally not be 
more files than the higher of wal_keep_segments or (2 + 
checkpoint_completion_target) * checkpoint_segments + 1. Each segment file is 
normally 16 MB (though this size can be altered when building the server). You 
can use this to estimate space requirements for WAL. Ordinarily, when old log 
segment files are no longer needed, they are recycled (renamed to become the 
next segments in the numbered sequence). If, due to a short-term peak of log 
output rate, there are more than 3 * checkpoint_segments + 1 segment files, the 
unneeded segment files will be deleted instead of recycled until the system 
gets back under this limit. 
"

Andres