Re: libata FUA revisited

From: Jens Axboe
Date: Wed Feb 21 2007 - 03:49:27 EST

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:
> >
> >
> >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


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

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