Re: [BK PATCHES] add ata scsi driver

Linus Torvalds
2003-05-26T20:37:07Z

On Mon, 26 May 2003, Jens Axboe wrote:
>
> I know this is a pet peeve of yours (must be, I remember you bringing it
> up at least 3 time before :), but I don't think that's necessarily true.
> It shouldn't matter _one_ bit whether you leave the request there or
> not, it's unmergeable.

It's not the merging that I worry about. It's latency and starvation.

Think of it this way: if you keep feeding a disk requests, and the disk
always tries to do the closest one (which is a likely algorithm), you can
easily have a situation where the disk _never_ actually schedules a
request that is at one "end" of the platter.

Think of all the fairness issues we've had in the elevator code, and
realize that the low-level disk probably implements _none_ of those
fairness algorithms.

> As long as the io scheduler keeps track of this (and it does!) we are
> golden.

Hmm.. Where does it keep track of request latency for requests that have
been removed from the queue?

Linus