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 - 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
