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From: Jens Axboe <jens.axboe@oracle.com>
To: Robert Hancock <hancockr@shaw.ca>
Cc: Tejun Heo <htejun@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-ide@vger.kernel.org, edmudama@gmail.com,
	Nicolas.Mailhot@LaPoste.net, Jeff Garzik <jeff@garzik.org>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>, Mark Lord <mlord@pobox.com>
Subject: Re: libata FUA revisited
Date: Wed, 21 Feb 2007 09:44:27 +0100	[thread overview]
Message-ID: <20070221084427.GA3924@kernel.dk> (raw)
In-Reply-To: <45D9FE7B.60909@shaw.ca>

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


  parent reply	other threads:[~2007-02-21  8:49 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <fa.S80SRyQbD/hm4SxliPUKU88BaCo@ifi.uio.no>
2007-02-12  5:47 ` Robert Hancock
     [not found] ` <fa.Q/csgyCHkAsD84yi+bN78H1WNNM@ifi.uio.no>
2007-02-13  0:23   ` Robert Hancock
2007-02-13 15:20     ` Tejun Heo
2007-02-14  0:07       ` Robert Hancock
2007-02-14  0:50         ` Tejun Heo
2007-02-15 18:00           ` Jens Axboe
2007-02-19 19:46             ` Robert Hancock
2007-02-21  8:37               ` Tejun Heo
2007-02-21  8:46                 ` Jens Axboe
2007-02-21  8:57                   ` Tejun Heo
2007-02-21  9:01                     ` Jens Axboe
2007-02-22 22:44                     ` Ric Wheeler
2007-02-22 22:40                   ` Ric Wheeler
2007-02-21 14:06                 ` Robert Hancock
2007-02-22 22:34                 ` Ric Wheeler
2007-02-23  0:04                   ` Robert Hancock
2007-02-21  8:44               ` Jens Axboe [this message]
2007-02-12  3:25 Robert Hancock
2007-02-12  8:31 ` Tejun Heo
2007-02-16 18:14   ` Jeff Garzik

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