Re: Database performs massive reads when I'm doing writes. - Mailing list pgsql-general

From: "bricklen" <bricklen@gmail.com>
To: "Ray Cote" <rgacote@appropriatesolutions.com>
Cc: pgsql-general@postgresql.org
Sent: Thursday, June 6, 2013 3:44:04 PM
Subject: Re: [GENERAL] Database performs massive reads when I'm doing writes.

On Thu, Jun 6, 2013 at 9:53 AM, Ray Cote <rgacote@appropriatesolutions.com> wrote:

Starting a few days ago we started to see a strange behavior where writing to the database causes massive read operations.
For example, I have a table that needs to be updated every night (about 20,000 rows).
Using Django ORM, we update them one item at a time.
Usually the overall process takes a few minutes; it is now taking hours (like over 15 hours).

Running atop, we're seeing Read Disk values in the range of 147.2M/10s and Write Disk values in the range of 16K/10s.
Together, the Disk throughput is in the high 90% and frequently hits 100%.

When I'm not writing to the database, it becomes quiet without any disk activity.

This is a 256 MByte system and atop shows we're not swapping

Is autovacuum set to 'on'?

Possibly there is bloat in your table and indexes; what happens to the system after you issue a manual "VACUUM ANALYZE VERBOSE"? If that helps, you might need to make your autovacuum settings more aggressive.