Re: libata FUA revisited

[Jens Axboe]

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

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