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Re: Load Distributed Checkpoints, take 3

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Load Distributed Checkpoints, take 3
Date: 2007-06-25 01:52:41
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-patches
On Sun, 24 Jun 2007, Simon Riggs wrote:

> I can't see why anyone would want to turn off smoothing: If they are
> doing many writes, then they will be effected by the sharp dive at
> checkpoint, which happens *every* checkpoint.

There are service-level agreement situations where a short and sharp 
disruption is more acceptable than a more prolonged one.  As some of the 
overloaded I/O tests are starting to show, the LDC may be a backward step 
for someone in that sort of environment.

I am not a fan of introducing a replacement feature based on what I 
consider too limited testing, and I don't feel this one has been beat on 
long yet enough to start pruning features that would allow better backward 
compatibility/transitioning.  I think that's introducing an unnecessary 
risk to the design.

> We won't need to set checkpoint_segments so high, since performance is 
> smoothed across checkpoints by LDC and its OK to allow them more 
> frequently. So this concern need not apply with LDC.

Performance *should* be smoothed across by checkpoints by LDC and my 
concern *may* not apply.  I think assuming it will always help based on 
the limited number of test results presented so far is extrapolation.

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

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