On Mon, Feb 19 2007, Robert Hancock wrote: > Jens Axboe wrote: > >But we can't really change that, since you need the cache flushed before > >issuing the FUA write. I've been advocating for an ordered bit for > >years, so that we could just do: > > > >3. w/FUA+ORDERED > > > >normal operation -> barrier issued -> write barrier FUA+ORDERED > > -> normal operation resumes > > > >So we don't have to serialize everything both at the block and device > >level. I would have made FUA imply this already, but apparently it's not > >what MS wanted FUA for, so... The current implementations take the FUA > >bit (or WRITE FUA) as a hint to boost it to head of queue, so you are > >almost certainly going to jump ahead of already queued writes. Which we > >of course really do not. > > I think that FUA was designed for a different use case than what Linux > is using barriers for currently. The advantage with FUA is when you have [snip] Yes that's pretty obvious, my point is just that FUA+ORDERED would be a nice thing to have for us. > >I'm not too nervous about the FUA write commands, I hope we can safely > >assume that if you set the FUA supported bit in the id AND the write fua > >command doesn't get aborted, that FUA must work. Anything else would > >just be an immensely stupid implementation. NCQ+FUA is more tricky, I > >agree that it being just a command bit does make it more likely that it > >could be ignored. And that is indeed a danger. Given state of NCQ in > >early firmware drives, I would not at all be surprised if the drive > >vendors screwed that up too. > > > >But, since we don't have the ordered bit for NCQ/FUA anyway, we do need > >to drain the drive queue before issuing the WRITE/FUA. And at that point > >we may as well not use the NCQ command, just go for the regular non-NCQ > >FUA write. I think that should be safe. > > Aside from the issue above, as I mentioned elsewhere, lots of NCQ drives > don't support non-NCQ FUA writes.. "Lots" meaning how many? All the ones I have here support FUA. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
