Re: linux deadline i/o elevator tuning

Jeff <threshar(at)torgo(dot)978(dot)org> wrote:

> If you have a halfway OK raid controller, CFQ is useless. You can
fire
> up something such as pgbench or pgiosim, fire up an iostat and then

> watch your iops jump high when you flip to noop or deadline and
> plummet on cfq.

An interesting data point, but not, by itself, conclusive. One of the
nice things about a good scheduler is that it allows multiple writes
to the OS to be combined into a single write to the controller cache.
I think that having a large OS cache and the deadline elevator allowed
us to use what some considered extremely aggressive background writer
settings without *any* discernible increase in OS output to the disk.
The significant measure is throughput from the application point of
view; if you see that drop as cfq causes the disk I/O to drop, *then*
you've proven your point.

Of course, I'm betting that's what you do see....

-Kevin