On Wed, Oct 05 2005, Steve Lord wrote:
> Jens Axboe wrote:
> >On Wed, Oct 05 2005, Andi Kleen wrote:
> >
> >>On Wednesday 05 October 2005 16:58, Jens Axboe wrote:
> >>
> >>
> >>>There are still unknowns, the HD still being the biggest one of course.
> >>>The problem is that you don't know the worst case HD performance, it
> >>>might be doing all sorts of rewriting, calibration, error correct etc
> >>>that can still screw you. So I think without definitely knowledge of
> >>>what the HD will do in case of errors (or a way to control that which
> >>>you definitely can on some drives), it's still pretty hazy. It gets
> >>>better, but if you are looking for complete guarantees I don't think
> >>>it's good enough.
> >>
> >>Yes, but GRIO has exactly the same problem. I assume they need custom
> >>calibration for each IO subsystem.
> >
> >
> >Indeed it does, and yes if they really want to provide the type of
> >guarantees that Steve listed, then that needs a custom box with either
> >custom or known disk firmware options. If not you cannot give absolute
> >guarantees and expect to always honor them. That's in addition to
> >anything you may need to change in software, if using Linux you would
> >need to audit/fix lots of things in the io path.
> >
>
> Definitely, from my memory, grio had a tool for measuring a disk subsystem.
> I think reality is closer to overspecing the hardware than to providing an
> absolute guarantee of bandwidth. Being able to prioritize individual I/O
> calls
> is just part of the picture.
The spec'ing part is the last piece in the puzzle, that will only tell
you what the io subsystem will typically do. It wont tell you how it
behaves in boundary or error conditions. It's trivial to say "oh the
disk does 35MiB/sec at the end zone, lets cap it at 20MiB/sec" and have
it work 99.9% of the time, the tricky part is the last 0.1%. Or whatever
percentage you want to assign to it :-)
--
Jens Axboe
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