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Re: Load distributed checkpoint V3

From: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: pgsql-patches(at)postgresql(dot)org
Subject: Re: Load distributed checkpoint V3
Date: 2007-03-26 00:46:47
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Lists: pgsql-hackerspgsql-patches
Greg Smith <gsmith(at)gregsmith(dot)com> wrote:

> > Here is the latest version of Load distributed checkpoint patch.
> Couple of questions for you:
> -Is it still possible to get the original behavior by adjusting your 
> tunables?  It would be nice to do a before/after without having to 
> recompile, and I know I'd be concerned about something so different 
> becoming the new default behavior.

Yes, if you want the original behavior, please set all of
checkpoint_[write|nap|sync]_percent to zero. They can be changed
at SIGHUP timing (pg_ctl reload). The new default configurations
are write/nap/sync = 50%/10%/20%. There might be room for discussion
in choice of the values.

> -Can you suggest a current test case to demonstrate the performance 
> improvement here?  I've tried several variations on stretching out 
> checkpoints like you're doing here and they all made slow checkpoint 
> issues even worse on my Linux system.  I'm trying to evaluate this fairly.

You might need to increase checkpoint_segments and checkpoint_timeout.
Here is the results on my machine:
I've set the values to 32 segs and 15 min to take advantage of it
in the case of pgbench -s100 then.

> -This code operates on the assumption you have a good value for the 
> checkpoint timeout.  Have you tested its behavior when checkpoints are 
> being triggered by checkpoint_segments being reached instead?

This patch does not work fully when checkpoints are triggered by segments.
Write phases still work because they refer to consumption of segments,
but nap and fsync phases only check amount of time. I'm assuming
checkpoints are triggered by timeout in normal use -- and it's my
recommended configuration whether the patch is installed or not.

ITAGAKI Takahiro
NTT Open Source Software Center

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