Re: Linux I/O tuning: CFQ vs. deadline

Josh Berkus wrote:
> FWIW, back when deadline was first introduced Mark Wong did some tests
> and found Deadline to be the fastest of 4 on DBT2 ... but only by about
> 5%.  If the read vs. checkpoint analysis is correct, what was happening
> is the penalty for checkpoints on deadline was almost wiping out the
> advantage for reads, but not quite.
>

Wasn't that before 8.3, where the whole checkpoint spreading logic
showed up?  That's really a whole different write pattern now than it
was then.  8.2 checkpoint writes were one big batch write amenable to
optimizing for throughput.  The new ones are not; the I/O is intermixed
with reads most of the time.

> Man, we'd need a lot of testing to settle this.  I guess that's why
> Linux gives us the choice of 4 ...
>

A recent on of these I worked on started with 4096 possible I/O
configurations we pruned down the most likely good candidates from.  I'm
almost ready to schedule a week on Mark's HP performance test system in
the lab now, to try and nail this down in a fully public environment for
once.

--
Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com  www.2ndQuadrant.com