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From: Jens Axboe <jens.axboe@oracle.com>
To: Tejun Heo <htejun@gmail.com>
Cc: Robert Hancock <hancockr@shaw.ca>,
	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>,
	Ric Wheeler <ric@emc.com>, Dongjun Shin <d.j.shin@samsung.com>,
	Hannes Reinecke <hare@suse.de>
Subject: Re: libata FUA revisited
Date: Wed, 21 Feb 2007 10:01:26 +0100	[thread overview]
Message-ID: <20070221090126.GC3924@kernel.dk> (raw)
In-Reply-To: <45DC0983.6000709@gmail.com>

On Wed, Feb 21 2007, Tejun Heo wrote:
> Jens Axboe wrote:
> > On Wed, Feb 21 2007, Tejun Heo wrote:
> >> [cc'ing Ric, Hannes and Dongjun, Hello.  Feel free to drag other people in.]
> >>
> >> 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.
> >> Yeah, I think if we have tagged write command and flush tagged (or
> >> barrier tagged) things can be pretty efficient.  Again, I'm much more
> >> comfortable with separate opcodes for those rather than bits changing
> >> the behavior.
> > 
> > ORDERED+FUA NCQ would still be preferable to an NCQ enabled flush
> > command, though.
> 
> I think we're talking about two different things here.
> 
> 1. The barrier write (FUA write) combined with flush.  I think it would
> help improving the performance but I think issuing two commands
> shouldn't be too slower than issuing one combined command unless it
> causes extra physical activity (moving head, etc...).

The command overhead is dwarfed by other factors, agree.

> 2. FLUSH currently flushes all writes.  If we can mark certain commands
> requiring ordering, we can selectively flush or order necessary writes.
>  (No need to flush 16M buffer all over the disk when only journal needs
> barriering)

Sure, anything is better than the sledge hammer flush. But my claim is
that an ORDERED+FUA enabled write for critical data would be a good
approach, and simple in software.

> >> Another idea Dongjun talked about while drinking in LSF was ranged
> >> flush.  Not as flexible/efficient as the previous option but much less
> >> intrusive and should help quite a bit, I think.
> > 
> > But that requires extensive tracking, I'm not so sure the implementation
> > of that for barriers would be very clean. It'd probably be good for
> > fsync, though.
> 
> I was mostly thinking about journal area.  Using it for other purposes
> would incur a lot of complexity.  :-(

Yep if it's just for the journal, the range is known and fixed, so the
flush range would work nicely there.

-- 
Jens Axboe


  reply	other threads:[~2007-02-21  9:02 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 [this message]
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
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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