Re: libata FUA revisited

[Jens Axboe]

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.

> 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.

-- 
Jens Axboe

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