Re: [BK PATCHES] add ata scsi driver

[Jens Axboe <axboe@suse.de>]

On Mon, May 26 2003, Linus Torvalds wrote:
> 
> 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. 

Then you have a bad disk, period. If the disks always tries to
approximate SPTF internally, then it's a bad design. Apparently that
Other OS times read/write requests out after 3 seconds, we we at least
know we are getting service in that time frame. Not saying that is good
enough, just a data point.

But the situation you describe above can easily be fixed, you described
the solution yourself in the previous mail. The silly tag depth is a
problem in itself, it should not be done. Keeping a sane number of tags
just to keep the disk busy, and we can use the "don't queue more
requests before X finishes, because X has been waiting for Y ms" tactic.

In fact, considering folks want to make error handling for command
timeouts a block property (that I agree with, we are already going there
with the SG_IO stuff), we can soft timeout a command if need be and
handle the case from there. What do you think?

> 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.

I think it does, to some extent at least.

> > 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?

Well, it doesn't...

-- 
Jens Axboe