Neil Brown (neilb@suse.de) wrote on 25 November 2010 07:01:
 >On Wed, 24 Nov 2010 22:34:21 +0800 Du Jun [off-list ref] wrote:
 >
 >> Their's no dmesg information at all before the hang. The system just 
 >> hangs suddenly without any kernel printk or oops.
 >> 
 >> mdadm version is debian lenny version 2.6.7.2-3
 >> 
 >> I will get mdadm -E & mdadm -D informations tomorrow but I doubt they 
 >> are useful.
 >
 >Probably not, but they wouldn't hurt either.
 >
 >What would really help is if you could capture the result of
 >    sysrq-T
 >
 >which I accept might be a challenge depending on how hard the machine really
 >has frozen.
 >
 >Also, make sure the LOCKUP_DETECTOR config options are set, and wait a few
 >minutes after the hand to see if something pops up.

You could also boot with the NMI watchdog. Using netconsole to capture
the full traces would be useful as well.