Regression with MSDOSFS in 4.0

[Rhialto <rhialto%falu.nl@localhost>]

For the first time since I installed 4.0 on my laptop I have the
occasion to use msdosfs and copy some large files to it.

I notice a regression: sequential write to a file gets slower as the
file gets longer. This was previously fixed with a patch in my
pr kern/34583
http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=34583 .

The code from the patch is actually present in the kernel, I checked
that. The problem is likely a change in the write-order of the write
cache. An earlier change like that had rendered the caching of FAT
entries ineffictive, and I strongly suspect the same thing has happened
again.

I can actually hear the disk seeking longer and longer distances
as the file grows, indicating it needs to look up the FAT again and
again as it writes the file data.

Can someone who actually knows something about the write-back order
shine some light on this?

Aside: the Firewire timeouts still don't seem to be fixed, also. This
slows the process down even more.

-Olaf.
-- 
___ Olaf 'Rhialto' Seibert      -- You author it, and I'll reader it.
\X/ rhialto/at/xs4all.nl        -- Cetero censeo "authored" delendum esse.