Re: Performance increase with elevator=deadline

* Jeff:

> Using 4 of these with a dataset of about 30GB across a few files
> (Machine has 8GB mem) I went from around 100 io/sec to 330 changing to
> noop. Quite an improvement. If you have a decent controller CFQ is
> not what you want. I tried deadline as well and it was a touch
> slower. The controller is a 3ware 9550sx with 4 disks in a raid10.
>
> I'll be trying this out on the big array later today. I found it
> suprising this info wasn't more widespread (the use of CFQ on a good
> controller).

3ware might be a bit special because the controller has got very deep
queues on its own, so many assumptions of the kernel I/O schedulers do
not seem to apply. Toying with the kernel/controller queue depths
might help, but I haven't done real benchmarks to see if it's actually
a difference.

A few days ago, I experienced this: On a machine with a 3ware
controller, a simple getxattr call on a file in an uncontended
directory took several minutes because a PostgreSQL dump process was
running in the background (and some other activity of a legacy
database which caused frequent fdatasync calls). Clearly, this is
unacceptable, and I've since switched to the deadline scheduler, too.
So far, this particular behavior hasn't occurred again.

--
Florian Weimer <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH http://www.bfk.de/
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