multiple pending filesystem requests for the same block ?

(context: SoC work on pluggable disk schedulers,
	http://wikitest.freebsd.org/moin.cgi/Hybrid


The doubt we have is the following:

	can the filesystem code have multiple pending write requests for
	the same disk block ?

the current (elevator) disk scheduler should have no problem with
this, because even if disksort reorders requests, two requests
for the same block retain the issue ordering.

However we wonder if this feature is actually expected by
the filesystem code, or it does not matter because it
never issues a new request before the previous one on the
same area is complete.

	thanks
	luigi