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Re: Streaming replication, and walsender during recovery

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication, and walsender during recovery
Date: 2010-01-28 16:22:10
Message-ID: 1264695730.24669.10882.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, 2010-01-28 at 10:48 -0500, Tom Lane wrote:
> Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
> > How about just making a restore_command copy the WAL files as the
> > normal one (e.g., 0000...) instead of a pg_xlog/RECOVERYXLOG?
> > Though we need to worry about deleting them, we can easily leave
> > the task to the bgwriter.
> 
> The reason for doing it that way was to limit disk space usage during
> a long restore.  I'm not convinced we can leave the task to the bgwriter
> --- it shouldn't be deleting anything at that point.

I think "bgwriter" means RemoveOldXlogFiles(), which would normally
clear down files at checkpoint. If that was added to the end of
RecoveryRestartPoint() to do roughly the same job then it could
potentially work.

However, since not every checkpoint is a restartpoint we might easily
end up with significantly more WAL files on the standby than would
normally be there when it would be a primary. Not sure if that is an
issue in this case, but we can't just assume we can store all files
needed to restart the standby on the standby itself, in all cases. That
might be an argument to add a restartpoint_segments parameter, so we can
trigger restartpoints on WAL volume as well as time. But even that would
not put an absolute limit on the number of WAL files.

I'm keen to allow cascading in 9.0. If you pull both synch rep and
cascading you're not offering much that isn't already there.

-- 
 Simon Riggs           www.2ndQuadrant.com


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