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Re: Refactoring standby mode logic

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Refactoring standby mode logic
Date: 2012-12-03 10:46:39
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Lists: pgsql-hackers
On 29 November 2012 09:06, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> wrote:
> The code that reads the WAL from the archive, from pg_xlog, and from a
> master server via walreceiver, is quite complicated. I'm talking about the
> WaitForWALToBecomeAvailable() function in xlog.c. I got frustrated with that
> while working on the "switching timeline over streaming replication" patch.
> Attached is a patch to refactor that logic into a more straightforward state
> machine. It's always been a kind of a state machine, but it's been hard to
> see, as the code wasn't explicitly written that way. Any objections?
> The only user-visible effect is that this slightly changes the order that
> recovery tries to read files from the archive, and pg_xlog, in the presence
> of multiple timelines. At the moment, if recovery fails to find a file on
> current timeline in the archive, it then tries to find it in pg_xlog. If
> it's not found there either, it checks if the file on next timeline exists
> in the archive, and then checks if exists in pg_xlog. For example, if we're
> currently recovering timeline 2, and target timeline is 4, and we're looking
> for WAL file A, the files are searched for in this order:
> 1. File 00000004000000000000000A in archive
> 2. File 00000004000000000000000A in pg_xlog
> 3. File 00000003000000000000000A in archive
> 4. File 00000003000000000000000A in pg_xlog
> 5. File 00000002000000000000000A in archive
> 6. File 00000002000000000000000A in pg_xlog
> With this patch, the order is:
> 1. File 00000004000000000000000A in archive
> 2. File 00000003000000000000000A in archive
> 3. File 00000002000000000000000A in archive
> 4. File 00000004000000000000000A in pg_xlog
> 5. File 00000003000000000000000A in pg_xlog
> 6. File 00000002000000000000000A in pg_xlog
> This change should have no effect in normal restore scenarios. It'd only
> make a difference if some files in the middle of the sequence of WAL files
> are missing from the archive, but have been copied to pg_xlog manually, and
> only if that file contains a timeline switch. Even then, I think I like the
> new order better; it's easier to explain if nothing else.

Sorry, forgot to say "fine by me".

This probably helps the avoidance of shutdown checkpoints, since for
that, we need to skip retrieving from archive once we're up.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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