HEADS-UP: RAIDframe parity map project to be committed

[Jed Davis <jld%panix.com@localhost>]

I'm about to commit my Summer of Code project, which drastically reduces
the time RAIDframe spends rewriting parity after an unclean shutdown by
keeping better track of outstanding writes (thus, "parity map").  The
tech-kern archives have more details, including reports of people
testing the changes; the relevant thread starts on 2009-10-20.

This feature is enabled by default on all sets (other than RAID 0).  It
can be administratively disabled with the new "raidctl -M" flag, which
is described in the changes to the raidctl(8) man page; however, the I/O
overhead for updating the parity map is expected to be unnoticeable in
practice.

As discussed in the tech-kern thread, the extensions to the RAID
metadata are backward and forward compatible, so mixing and matching
kernels from before and after this change should Do The Right Thing.

-- 
(let ((C call-with-current-continuation)) (apply (lambda (x y) (x y)) (map
((lambda (r) ((C C) (lambda (s) (r (lambda l (apply (s s) l))))))  (lambda
(f) (lambda (l) (if (null? l) C (lambda (k) (display (car l)) ((f (cdr l))
(C k)))))))    '((#\J #\d #\D #\v #\s) (#\e #\space #\a #\i #\newline)))))