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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
