Re: Poor floppy performance in kernel 2.4.10

[Kamil Iskra]

On Thu, 18 Oct 2001, Andreas Dilger wrote:

> > The behaviour is as if no caching was done,
> > there is a slowdown by a factor of two.
> I think this is a result of the "blockdev in pagecache" change added in
> 2.4.10.  One of the byproducts of this change is that if a block device
> is closed (no other openers) then all of the pages from this device are
> dropped from the cache.  In the case of a floppy drive, this is very
> important, as you don't want to be cacheing data from one floppy after
> you have inserted a new floppy.
>
> In contrast, if you mounted the floppy instead of using mtools, it would
> probably have good performance for small files as well.

That's very interesting.  It would explain why it takes 2 seconds _every_
time you invoke "mdir", whereas before the invocations after the first one
were more or less instantenous.  And indeed, as you say, mounting a floppy
does result in a good performance.

However, it does not explain why the first invocation is two times slower
(it's 1 sec with kernel 2.4.9 and 2 secs with 2.4.10, the effect is even
more visible for mcopy of a small file, like 30KB).  I strace'd mdir and
it's opening /dev/fd0 just once, at the beginning, and closing it at the
end.

Regards,

-- 
Kamil Iskra                 http://www.science.uva.nl/~kamil/
Section Computational Science, Faculty of Science, Universiteit van Amsterdam
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