MD(4)/vnode hangs resolved (?)

Those of you who have experienced that md(4) backed by vnodes hang your
systems, please try this patch:

	http://phk.freebsd.dk/patch/md.patch

It changes so that vnode backed md(4) devices use IO_SYNC by default
(can be overridden with "async" option to mdconfig(8)).

This solves the case where the system get stuck in "wdrain" because
a md(4)/vnode device is relatively large compared to the buffer cache.

Another benefit is that this makes the md(4)/vnode devices behave like
a proper disk with no write-caching.

Performance sucks though.

But slow performance is still better than hung systems.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk_at_FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.