Re: raid and elevator

[Jens Axboe <axboe@suse.de>]

On Sun, Mar 26 2006, Raz Ben-Jehuda(caro) wrote:
> It is my belief that the linux elevators are not optimized for
> raid throughout.
> 
> Basically when IO initiator generates IO over a single disk he can control
> the amount of IOs and this way control IOs latencies.
> 
> This is not the case in raided volumes.
> 
> Example:
> 
> An IO initiator intiates bulks of asynchronous IOs over a raided
> volume. since the user does not know the target destination of each IO
> he
> might cause imbalance over the disks. one disk may become overflooded
> with IOs while the other is may starve. So eventually we are ending with
> a non balaned raided volume in terms of bad latencies.
> 
> I am trying to deal with this problem by fixing the deadline elevator code
> to batch IOs , meaning , when n IOs are reaching the disk, each m deadlined
> IOs are sorted and then dispatced.

You do realize that the io scheduler resides _below_ the raid
personality? So if you want to balance what goes to what io scheduler
(and thus, disk), you'd want to mess with the raid personality.

-- 
Jens Axboe