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SubjectRe: SCSI TMF processing; tag allocation
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On Mon, 2010-11-15 at 07:46 -0700, Matthew Wilcox wrote:
> On Mon, Nov 15, 2010 at 08:33:15AM -0600, James Bottomley wrote:
> > Right, it's the clock algorithm to prevent tag starvation. If you have
> > hands representing the first and last tag and they're never allowed to
> > cross, the device can't starve any tag for too long because eventually
> > it will be the only outstanding command.
> >
> > It's not the only algorithm however. Banging down an ordered tag every
> > 200 or so commands has exactly the same effect. In fact the clock
> > algorithm was what the 53c700 driver used (before it was converted to
> > generic tags) and the ordered tag what aic7xxx uses.
> >
> > Realistically, tag starvation isn't really a problem. It was a known
> > issue for 80s era hardware. I've got some of the oldest drives on the
> > planet and I didn't see a problem when the clock algorithm was removed
> > from 53c700.
>
> The problem is that each driver is solving the problem in its own way
> right now, which is clearly daft. And no drive manufactured in the past
> fifteen years supports ordered tags anyway, so they're only a placebo
> at this point.

Actually, most drivers ignore the issue ... which is why I don't think
there's much of a problem. The aic7xxx is the only one I know doing
something about this. Like I said, I removed the clock algorithm from
53c700 and haven't had a problem (yet, I suppose).

James




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