Re: barriers vs. reads - O_DIRECT

[Werner Almesberger <wa@almesberger.net>]

Bryan Henderson wrote:
> It seems obvious to me that whatever ordering guarantees the user gets 
> without the O_DIRECT flag, he should get with it as well.

Yes, it would be nice if we could obtain such behaviour without
unacceptable performance sacrifices. It seems to me that, if we
can find an efficient way for serializing all write-write and
read-write overlaps, plus have explicit barriers for serializing
non-overlapping writes, this should yield pretty much what
everyone wants (*). Now, that "if" needs a bit of work ... :-)

(*) The only difference being that a completing read doesn't
    tell you whether the elevator has already passed a barrier.
    Currently, one could be lured into depending on this.

> When I worry about I/O scheduling, I usually worry a lot more about block 
> device direct I/O (raw devices) than file direct I/O.  The former is where 
> the block layer is most exposed to the user.

I haven't looked at the direct IO code in detail, but it seems
to me that the elevator behaviour affects file-based direct IO
in the same way as the device-based one.

- Werner

-- 
  _________________________________________________________________________
 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
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