Subject: Re: Simplifying replication
From: Simon Riggs <simon@2ndQuadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Greg Stark <gsstark@mit.edu>, Josh Berkus <josh@agliodbs.com>,
	Bruce Momjian <bruce@momjian.us>, pgsql-hackers@postgresql.org
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Date: Tue, 26 Oct 2010 13:27:29 +0100
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On Thu, 2010-10-21 at 20:57 -0400, Robert Haas wrote:

> Very true.  But the lack of a -1 setting for wal_keep_segments means
> that if you would like to take a backup without archiving, you must
> set wal_keep_segments to a value greater than or equal to the rate at
> which you generate WAL segments multiplied by the time it takes you to
> run a backup.  If that doesn't qualify as requiring arcane knowledge,
> I'm mystified as to what ever could.

People are missing the point here:

You have to put the WAL files *somewhere* while you do the base backup.
PostgreSQL can't itself work out where that is, nor can it work out
ahead of time how big it will need to be, since it is up to you how you
do your base backup. Setting a parameter to -1 doesn't make the problem
go away, it just pretends and hopes it doesn't exist, but screws you
badly if you do hit the wall. 

My view is that is irresponsible, even if I share people's wish that the
problem did not exist.

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services