Messages in this thread | | | Date | Mon, 26 May 2003 22:38:00 +0200 | From | Jens Axboe <> | Subject | Re: [BK PATCHES] add ata scsi driver |
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On Mon, May 26 2003, Linus Torvalds wrote: > > On 26 May 2003, James Bottomley wrote: > > > > On Mon, 2003-05-26 at 15:07, Jens Axboe wrote: > > > Alright, so what do you need? Start out with X tags, shrink to Y (based > > > on repeated queue full conditions)? Anything else? > > > > Actually, it's easier than that: just an API to alter the number of tags > > in the block layer (really only the size of your internal hash table). > > The actual heuristics of when to alter the queue depth is the province > > of the individual drivers (although Doug Ledford was going to come up > > with a generic implementation). > > Talking about tagged queueing - does the SCSI layer still remove the > request from the request list when it starts executing it?
Yes
> At least historically that's a major mistake, and generates a crappy > elevator, because it removes information from the block layer about where > the disk is (or is going to be).
Not necessarily, the io schedulers keep track of where we are going. You don't need an active front request for that.
> I know Andrew thinks that SCSI tagged queuing is a bunch of crap, and he > has the latency numbers to prove it. He blames the SCSI disks themselves, > but I think it might be the fact that SCSI makes it impossible to make a > fair queuing algorithm for higher levels by hiding information. > > Has anybody looked at just removing the request at command _completion_ > time instead? That's what IDE does, and it's the _right_ thing to do.
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. As long as the io scheduler keeps track of this (and it does!) we are golden.
-- Jens Axboe
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