Re: Slow queries / commits, mis-configuration or hardware issues?

Thanks for your response. Please see below for answers to your questions.

On Mon, Nov 14, 2011 at 11:22 AM, Tomas Vondra <tv@fuzzy.cz> wrote:
> On 14 Listopad 2011, 19:16, Cody Caughlan wrote:
>> shared_buffers = 3584MB
>> wal_buffers = 16MB
>> checkpoint_segments = 32
>> max_wal_senders = 10
>> checkpoint_completion_target = 0.9
>> wal_keep_segments = 1024
>> maintenance_work_mem = 256MB
>> work_mem = 88MB
>> shared_buffers = 3584MB
>> effective_cache_size = 10GB
>
> Seems reasonable, although I'd bump up the checkpoint_timeout (the 5m is
> usually too low).

Ok, will do.

>
>> The PGDATA dir is a RAID10 on 4 local ("ephemeral" in EC2 speak)
>> drives. I ran some dd tests and received the following output:
>>
>> --- WRITING ---
>> root@sql03:/data# time sh -c "dd if=/dev/zero of=/data/tmp/bigfile
>> bs=8k count=4000000 && sync"
>> 4000000+0 records in
>> 4000000+0 records out
>> 32768000000 bytes (33 GB) copied, 670.663 s, 48.9 MB/s
>>
>> real  11m52.199s
>> user  0m2.720s
>> sys   0m45.330s
>
> This measures sequential write performance (and the same holds for the
> read test). We need to know the random I/O performance too - use bonnie++
> or similar tool.
>
> Based on the AWS benchmarks I've seen so far, I'd expect about 90 MB/s for
> sequential read/writes, and about twice that performance for a 4-drive
> RAID10. So while the reads (211 MB/s) seem perfectly OK, the writes
> (50MB/s) are rather slow. Have you measured this on an idle system, or
> when the db was running?
>

I ran bonnie++ on a slave node, doing active streaming replication but
otherwise idle:
http://batch-files-test.s3.amazonaws.com/sql03.prod.html

bonnie++ on the master node:
http://batch-files-test.s3.amazonaws.com/sql01.prod.html

If I am reading this right, this is my first time using it, the
numbers dont look too good.

> See for example this:
>
> [1] http://victortrac.com/EC2_Ephemeral_Disks_vs_EBS_Volumes
> [2]
> http://www.gabrielweinberg.com/blog/2011/05/raid0-ephemeral-storage-on-aws-ec2.html
>
>> I have enabled log_checkpoints and here is a recent sample from the log:
>> 2011-11-14 17:39:15 UTC pid:3965 (-0) LOG:  checkpoint starting: time
>> 2011-11-14 17:43:49 UTC pid:3965 (-0) LOG:  checkpoint complete: wrote
>> 16462 buffers (3.6%); 0 transaction log file(s) added, 0 removed, 9
>> recycled; write=269.978 s, sync=4.106 s, total=274.117 s; sync
>> files=82, longest=2.943 s, average=0.050 s
>
> Nothing special here - this just says that the checkpoints were timed and
> finished on time (the default checkpoint timeout is 5 minutes, with
> completion target 0.9 the expected checkpoint time is about 270s). Not a
> checkpoint issue, probably.
>
>> I've been collecting random samples from pg_stat_bgwriter:
>> https://gist.github.com/4faec2ca9a79ede281e1
>
> Although it's a bit difficult to interpret this (collect the data in
> regular intervals - e.g. every hour - and post the differences, please),
> but it seems reasonable.

Ok, I have a cron running every hour to grab this data. I will post
back in a few hours or tomorrow.

>
>> So given all this information (if you need more just let me know), is
>> there something fundamentally wrong or mis-configured? Do I have an
>> I/O issue?
>
> Probably - the discrepancy between read/write performance is a bit
> suspicious.
>
> Try to watch the I/O performance when this happens, i.e. run "iostat -x"
> and watch the output (especially %util, r_await, w_await) and post several
> lines of the output.
>

Heres a gist of running "iostat -x 3" for about a few minutes:

https://gist.github.com/f94d98f2ef498a522ac2

Indeed, the %iowat and await values can spike up drastically.