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, 5 insertions, 1 deletions

diff --git a/sbin/mdconfig/mdconfig.c b/sbin/mdconfig/mdconfig.c
index 6296da7..b23e903 100644
--- a/sbin/mdconfig/mdconfig.c
+++ b/sbin/mdconfig/mdconfig.c
@@ -117,7 +117,11 @@ main(int argc, char **argv)
 		case 'o':
 			if (cmdline != 2)
 				usage();
-			if (!strcmp(optarg, "cluster"))
+			if (!strcmp(optarg, "async"))
+				mdio.md_options |= MD_ASYNC;
+			else if (!strcmp(optarg, "noasync"))
+				mdio.md_options &= ~MD_ASYNC;
+			else if (!strcmp(optarg, "cluster"))
 				mdio.md_options |= MD_CLUSTER;
 			else if (!strcmp(optarg, "nocluster"))
 				mdio.md_options &= ~MD_CLUSTER;