Message-ID: <4CC8931D.3040800@agliodbs.com>
Date: Wed, 27 Oct 2010 14:01:17 -0700
From: Josh Berkus <josh@agliodbs.com>
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US;
	rv:1.9.1b3pre) Gecko/20090223 Thunderbird/3.0b2
MIME-Version: 1.0
To: Simon Riggs <simon@2ndQuadrant.com>
CC: Robert Haas <robertmhaas@gmail.com>, Greg Stark <gsstark@mit.edu>,
	Bruce Momjian <bruce@momjian.us>, pgsql-hackers@postgresql.org
Subject: Re: Simplifying replication
References: <201010220022.o9M0M8Q29438@momjian.us>
	<4CC0DEDE.7060707@agliodbs.com>
	<AANLkTin0RfGOvk20vLYYfCDzdf5Sf94aQ5V-xOBB+Kqx@mail.gmail.com>
	<AANLkTik-L-68UpwgyBh5HtROzkEYQ297fiy7C-c6qRfX@mail.gmail.com>
	<1288096049.1587.253.camel@ebony>
In-Reply-To: <1288096049.1587.253.camel@ebony>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit


> 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. 

Agreed.  That's why I like the idea of having a
max_wal_size/min_wal_time instead of keep_wal_segments or
checkpoint_segments.  It's relatively simple for a DBA to know how much
disk space s/he has for WAL, total, before locking up the system.

And to answer Robert's question, because now I understand what he was
getting at.  The reason we want a min_wal_time is because we don't want
to keep a larger WAL around always.  If more WAL were always better,
then we'd only need max_wal_size and we'd only recycle when we hit it.
Instead, we'd recycle whenever we passed max_wal_time.  That's why I
said that I was assuming nothing of the sort.

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com