From: Ric Wheeler <firstname.lastname@example.org> To: Tejun Heo <email@example.com> Cc: Jens Axboe <firstname.lastname@example.org>, Robert Hancock <email@example.com>, linux-kernel <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, Nicolas.Mailhot@LaPoste.net, Jeff Garzik <email@example.com>, Alan Cox <firstname.lastname@example.org>, Mark Lord <email@example.com>, Dongjun Shin <firstname.lastname@example.org>, Hannes Reinecke <email@example.com> Subject: Re: libata FUA revisited Date: Thu, 22 Feb 2007 17:44:59 -0500 [thread overview] Message-ID: <45DE1CEB.firstname.lastname@example.org> (raw) In-Reply-To: <45DC0983.email@example.com> 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...). > > 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) We can certainly (given time to play in the lab!) try to measure this in with a micro-benchmark (with an analyzer or with block trace?). A normal flush command in my old tests seemed to be in the 20 ms range (mixed in with and occasional "freebie" cache flush which returns in 50 usecs or so - cache must be empty). >>> 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. :-( >
next prev parent reply other threads:[~2007-02-22 22:46 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 [this message] 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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