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Re: Simplifying replication

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Simplifying replication
Date: 2010-10-27 21:01:17
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Lists: pgsql-hackers
> 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.

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