From: | Greg Smith <gsmith(at)gregsmith(dot)com> |
---|---|
To: | |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: lowering impact of checkpoints |
Date: | 2007-09-25 22:21:10 |
Message-ID: | Pine.GSO.4.64.0709251753310.2193@westnet.com |
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Thread: | |
Lists: | pgsql-general |
On Tue, 25 Sep 2007, Gregory Stark wrote:
> I'm surprised you don't start by suggesting lowering bgwriter_delay for a busy
> dedicated system. Does it cause too much wasted cpu work in the "all" cycle in
> 8.2?
I've just found it easier to sort through this class of problem by getting
the maxpages parameters into at least the 200-500 range before even
thinking about lowering the delay. There may very well be a different way
to approach this problem by making the delay more of a primary tunable.
Certainly there's potentially an advantage to lowering the delay in that
it gets writes trickling out to disk more regularly.
> I also wonder if it doesn't make more sense in 8.2 if your goal is to avoid
> drop-outs to just give up on the lru cycle entirely and set the delay to
> something like 60s and the all_percent to 100.
There are some workloads where flushing the buffers that haven't been used
recently in the lru cycle is more useful than what the all scan does; it's
hard to figure out whether your system is such a case or not in 8.2
though.
In addition, the main problem with using a longer cycle/higher percentage
is that the way some operating systems buffer writes favors writing small
blocks more frequently. In the Linux case there are situations where
writes sit there for a full 30 seconds so getting the physical disk
started earlier is a benefit. I'd be concerned that all_percent=100 would
end up generating something close to a checkpoint I/O spike every cycle,
and that the background writer waiting for that big write to finish might
delay checkpoint requests from processing in a timely fashion.
--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD
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