Fix a long-standing deadlock issue with vnode backed md(4) devices:

On vnode backed md(4) devices over a certain, currently undetermined
size relative to the buffer cache our "lemming-syncer" can provoke
a buffer starvation which puts the md thread to sleep on wdrain.

This generally tends to grind the entire system to a stop because the
event that is supposed to wake up the thread will not happen until a fair
bit of the piled up I/O requests in the system finish, and since a lot
of those are on a md(4) vnode backed device which is currently waiting
on wdrain until a fair amount of the piled up ... you get the picture.

The cure is to issue all VOP_WRITES on the vnode backing the device
with IO_SYNC.

In addition to more closely emulating a real disk device with a
non-lying write-cache, this makes the writes exempt from rate-limited
(there to avoid starving the buffer cache) and consequently prevents
the deadlock.

Unfortunately performance takes a hit.

Add "async" option to give people who know what they are doing the
old behaviour.

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/sys/mdioctl.h b/sys/sys/mdioctl.h
index 3ba206b..861619b 100644
--- a/sys/sys/mdioctl.h
+++ b/sys/sys/mdioctl.h
@@ -90,5 +90,6 @@ struct md_ioctl {
 #define MD_READONLY	0x08	/* Readonly mode */
 #define MD_COMPRESS	0x10	/* Compression mode */
 #define MD_FORCE	0x20	/* Don't try to prevent foot-shooting */
+#define MD_ASYNC	0x40	/* Don't try to prevent foot-shooting */
 
 #endif	/* _SYS_MDIOCTL_H_*/