Since this discussion has forked into two discussions, I want to ask
Manuel about his idea for write ordering with disksort() and the sluggish
behavior one sees with machines which perform heavy i/o to the same disk on
which paging activities are taking place.
	It strikes me that one simple way to fix this problem, without destroying
the integrity in which things are written, is to tag each write request
with an identifier as to who the request came from.  Then, all requests
from the VM subsystem could be put at the beginning of the queue, while
normal ordering techniques would be used for all other requests.
My questions are:

1.  Is there a simple mechanism for tagging who a write request came from?

2.  Are there complexities here that I just don't understand with regard to
write ordering?

3.  What about the note in /usr/src/sys/scsipi/sd.c which says that
disksort() should only be called if the disk isn't running in tag-queueing
mode, just before disksort() is unconditionally called anyway?  

	Just my thoughts, if anyone would care to enlighten me, I'd be most 
appreciative.

-thanks
-Brian