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Re: Load Distributed Checkpoints, final patch

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Load Distributed Checkpoints, final patch
Date: 2007-06-28 13:22:19
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Lists: pgsql-patches
On Wed, 27 Jun 2007, Tom Lane wrote:

> Also, the question of redesigning the bgwriter's LRU scan is
> still open.  I believe that's on Greg's plate, too.

Greg's plate was temporarily fried after his house was hit by lightening 
yesterday.  I just got everything back on-line again, so no coding 
progress, but I think I finished assimilating your "epiphany" during that 
time.  Now I realize that what you're suggesting is that under healthy 
low-load conditions, the LRU really should be able to keep up right behind 
the clock sweep point.  Noting how far behind it is serves as a 
measurement of it failing to match the rate buffers that could be re-used 
are being dirtied, and the only question is how fast and far it should try 
to drive the point it has cleaned to forward when that happens.

> Once you've built up enough XLOG segments, the system isn't too bad 
> about recycling them, but there will be a nasty startup transient where 
> foreground processes have to stop and make the things.

Exactly.  I found it problematic in four situations:

1) Slow checkpoint doesn't finish in time and new segments are being 
created while the checkpoint is also busy (this is the really bad one)

2) Archive logger stop doing anything (usually because the archive disk is 
filled) and nothing gets recycled until that's fixed.

2) checkpoint_segments is changed, so then performance is really sluggish 
for a bit until all the segments are built back up again

3) You ran an early manual checkpoint which doesn't seem to recycle as 
many segments usefully

Any change that would be more proactive about creating segments in these 
situations than the current code would be a benefit, even though these are 
not common paths people encounter.

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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