On 12/07/2011 16:18, Merlin Moncure wrote:
On Tue, Jul 12, 2011 at 8:22 AM, Ivan Voras<ivoras@xxxxxxxxxxx> wrote:
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 :) ).
sure, but the OP's question is valid: in postgres, readers don't block
writers, so why is the reader waiting?
Yes, but I'm suggesting a different question: are we sure we are not
seeing the influences of the environment (EC2+EBS) instead of the
software system?
We need some more information here.
Definitely.
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