Re: Bandwidth Allocations under CFQ I/O Scheduler

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Oct 18 2006, Helge Hafting wrote:
> Jens Axboe wrote:
> >While that may make some sense internally, the exported interface would
> >never be workable like that. It needs to be simple, "give me foo kb/sec
> >with max latency bar for this file", with an access pattern or assumed
> >sequential io.
> >
> >Nobody speaks of iops/sec except some silly benchmark programs. I know
> >that you are describing pseudo-iops, but it still doesn't make it more
> >clear.
> >Things aren't as simple
> >  
> How about "give me 10% of total io capacity?"  People understand
> this, and the io scheduler can then guarantee this by ensuring
> that the process gets 1 out of 10 io requests as long as it
> keeps submitting enough.

The thing about disks is that it's not as easy as giving the process 10%
of the io requests issued. Only if the considered bandwidth is random
load will that work, but that's not very interesting.

You need to say 10% of the disk time, which is something CFQ can very
easily be modified to do since it works with time slices already. 10%
doesn't mean very much though, you need a timeframe for that to make
sense anyways. Give me 100msec every 1000msecs makes more sense.

-- 
Jens Axboe