Re: Optimize update query

On 11/28/2012 10:19 AM, Niels Kristian Schjødt wrote:

> https://rpm.newrelic.com/public/charts/h2dtedghfsv

Doesn't this answer your question?

That iowait is crushing your server into the ground. It's no surprise
updates are taking several seconds. That update you sent us *should*
execute on the order of only a few milliseconds.

So I'll reiterate that you *must* move your pg_xlog location elsewhere.
You've got row lookup bandwidth conflicting with writes. There are a
couple other changes you should probably make to your config:

> checkpoint_segments = 16

This is not enough for the workload you describe. Every time the
database checkpoints, all of those changes in pg_xlog are applied to the
backend data files. You should set these values:

checkpoint_segments = 100
checkpoint_timeout = 10m
checkpoint_completion_target = 0.9

This will reduce your overall write workload, and make it less active.
Too many checkpoints massively reduce write throughput. With the
settings you have, it's probably checkpointing constantly while your
load runs. Start with this, but experiment with increasing
checkpoint_segments further.

If you check your logs now, you probably see a ton of "checkpoint
starting: xlog" in there. That's very bad. It should say "checkpoint
starting: time" meaning it's keeping up with your writes naturally.

> work_mem = 160MB

This is probably way too high. work_mem is used every sort operation in
a query. So each connection could have several of these allocated, thus
starting your system of memory which will reduce that available for page
cache. Change it to 8mb, and increase it in small increments if necessary.

> So correct me if I'm wrong here: my theory is, that I have too many
> too slow update queries, that then often end up in a situation, where
> they "wait" for each other to finish, hence the sometimes VERY long
> execution times.

Sometimes this is the case, but for you, you're running into IO
contention, not lock contention. Your 3TB RAID-1 is simply insufficient
for this workload.

If you check your logs after making the changes I've suggested, take a
look at your checkpoint sync times. That will tell you how long it took
the kernel to physically commit those blocks to disk and get a
confirmation back from the controller. If those take longer than a
second or two, you're probably running into controller buffer overflows.
You have a large amount of RAM, so you should also make these two kernel
changes to sysctl.conf:

vm.dirty_ratio = 10
vm.dirty_writeback_ratio = 1

Then run this:

sysctl -p

This will help prevent large IO write spikes caused when the kernel
decides to write out dirty memory. That can make checkpoints take
minutes to commit in some cases, which basically stops all write traffic
to your database entirely.

That should get you going, anyway. You still need more/better disks so
you can move your pg_xlog directory. With your write load, that will
make a huge difference.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas@optionshouse.com

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