Linux filesystem performance and checkpoint sorting

Switching to a new thread for this summary since there's some much more
generic info here...at this point I've finished exploring the major
Linux filesystem and tuning options I wanted to, as part of examining
changes to the checkpoint code. You can find all the raw data at
http://www.2ndquadrant.us/pgbench-results/index.htm Here are some
highlights of what's been demonstrated there recently, with a summary of
some of the more subtle and interesting data in the attached CSV file too:

-On ext3, tuning the newish kernel tunables dirty_bytes and
dirty_background_bytes down to a lower level than was possible using the
older dirty_*ratio ones shows a significant reduction in maximum latency
on ext3; it drops to about 1/4 of the worst-case behavior.
Unfortunately transactions per second takes a 10-15% hit in the
process. Not shown in the data there is that the VACUUM cleanup time
between tests was really slowed down, too, running at around half the
speed of when the system has a full-size write cache.

-Switching from ext3 to xfs gave over a 3X speedup on the smaller test
set: from the 600-700 TPS range to around 2200 TPS. TPS rate on the
larger data set actually slowed down a touch on XFS, around 10%. Still,
such a huge win when it's better makes it easy to excuse the occasional
cases where it's a bit slower. And the latency situation is just wildly
better, the main thing that drove me toward using XFS more in the first
place. Anywhere from 1/6 to 1/25 of the worst-case latency seen on
ext3. With abusively high client counts for this hardware, you can
still see >10 second pauses, but you don't see >40 second ones at
moderate client counts like ext3 experiences.

-Switching to the lower possible dirty_*bytes parameters on XFS was
negative in every way. TPS was cut in half, and maximum latency
actually went up. Between this and the nasty VACUUM slowdown, I don't
really see that much potential for these new tunables. They do lower
latency on ext3 a lot, but even there the penalty you pay for that is
quite high. VACUUM in particular seems to really, really benefit from
having a giant write cache to dump its work into--possibly due to the
way the ring buffer implementation avoids using the database's own cache
for that work.

-Since earlier tests suggested sorting checkpoints gave little change on
ext3, I started testing that with XFS instead. The result is a bit
messy. At the lower scale, TPS went up a bit, but so did maximum
latency. At the higher scale, TPS dropped in some cases (typically less
than 1%), but most latency results were better too.

At this point I would say checkpoint sorting remains a wash: you can
find workloads it benefits a little, and others it penalizes a little.
I would say that it's neutral enough on average that if it makes sense
to include for other purposes, that's unlikely to be a really bad change
for anyone. But I wouldn't want to see it committed by itself; there
needs to be some additional benefit from the sorting before it's really
worthwhile.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
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