Re: UPDATEDs slowing SELECTs in a fully cached database

On 08/07/2011 01:56, lars wrote:

> Setup:
> PostgreSQL 9.1beta2 on a high memory (~68gb, 12 cores) EC2 Linux
> instance (kernel 2.6.35) with the database and
> WAL residing on the same EBS volume with EXT4 (data=ordered, barriers=1)
> - yes that is not an ideal setup
> (WAL should be on separate drive, EBS is slow to begin, etc), but I am
> mostly interested in read performance for a fully cached database.

I know you said you know these things - but do you really know the
(huge) extent to which all your IO is slowed? Even context switches in a
virtualized environment are slowed down by a huge margin - which would
make practically all in-memory lock operations very slow - much slower
than they would be on "real" hardware, and EBS by definition is even
slower then regular private virtual storage environments. I regrettably
didn't bookmark the page which did exact measurements of EBS, but
http://www.google.com/search?q=how+slow+is+ec2 will illustrate my point.
(of course, you may already know all this :) ).