kern/154228: [md] md getting stuck in wdrain state

[Carl]

The following reply was made to PR kern/154228; it has been noted by GNATS.

From: Carl <k0802647 at telus.net>
To: bug-followup at FreeBSD.org, k0802647 at telus.net
Cc:  
Subject: Re: kern/154228: [md] md getting stuck in wdrain state
Date: Tue, 08 Feb 2011 02:59:41 -0800

 For whatever reason I was not copied on the patch message, despite being 
 the bug reporter.
 
 The explanation for that patch is more than a little obscure. In simpler 
 terms, what have you uncovered?
 
 Does that patch implement a complete fix, partial fix, a workaround, or 
 what? Is it recommended I try it?
 
 Did someone manage to reproduce my problem scenario?
 
 Yesterday I ran into the same bug. Similar but different exercise. Again 
 on a remote production system. I had no choice but to try again, so I 
 repeated the procedure, only using a non-sparse file instead. It hung 
 yet again, so that should rule out sparse files as part of the problem.
 
 I noticed in the mdconfig(8) man page this description for the "-o 
 [no]async" option:
 
    'For vnode backed devices: avoid IO_SYNC for increased
     performance but at the risk of deadlocking the entire
     kernel.'
 
 It seems to me the default would be "-o noasync" and that this is 
 supposed to avoid that particular risk for deadlock, but what command 
 can I use to verify whether a particular enabled memory disk is actually 
 using IO_SYNC or not?
 
 Carl                                                   / K0802647