Re: Optimize update query - Mailing list pgsql-performance

On 11/29/2012 08:32 PM, Niels Kristian Schjødt wrote:

> If I do a "sudo iostat -k 1"
> I get a lot of output like this:
> Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
> sda               0.00         0.00         0.00          0          0
> sdb               0.00         0.00         0.00          0          0
> sdc             546.00      2296.00      6808.00       2296       6808
> sdd             593.00      1040.00      7416.00       1040       7416
> md1               0.00         0.00         0.00          0          0
> md0               0.00         0.00         0.00          0          0
> md2            1398.00      3328.00     13064.00       3328      13064
> md3               0.00         0.00         0.00          0          0
>

> The storage thing is, that the sda and sdb is the SSD drives and the
> sdc and sdd is the HDD drives. The md0, md1 and md2 is the raid
> arrays on the HDD's and the md3 is the raid on the SSD's. Neither of
> the md3 or the SSD's are getting utilized - and I should expect that
> since they are serving my pg_xlog right?

No, that's right. They are, but it would appear that the majority of
your traffic actually isn't due to transaction logs like I'd suspected.
If you get a chance, could you monitor the contents of:

/var/lib/postgresql/9.2/main/base/pgsql_tmp

Your main drives are getting way, way more writes than they should. 13MB
per second is ridiculous even under heavy write loads. Based on the TPS
count, you're basically saturating the ability of those two 3TB drives.
Those writes have to be coming from somewhere.

> #   sudo mkdir -p /ssd/pg_xlog

This is going to sound stupid, but are you *sure* the SSD is mounted at
/ssd ?

> #   sudo chown -R  postgres.postgres /ssd/pg_xlog
> #   sudo chmod 700 /ssd/pg_xlog
> #   sudo cp -rf /var/lib/postgresql/9.2/main/pg_xlog/* /ssd/pg_xlog
> #   sudo mv /var/lib/postgresql/9.2/main/pg_xlog /var/lib/postgresql/9.2/main/pg_xlog_old
> #   sudo ln -s /ssd/pg_xlog /var/lib/postgresql/9.2/main/pg_xlog
> #   sudo /etc/init.d/postgresql-9.2 start

The rest of this is fine, except that you probably should have added:

sudo chown -R postgres:postgres /ssd/pg_xlog/*


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

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