ffs snapshot lockup

[Vivek Khera]

Yesterday we upgraded an amd64 system from 6.1 to 6.2-PRE from a  
cvsup of September 20 evening.  The overnight nightly dump to tape  
went off normally.  This afternoon when the sysadmin was performing  
the level0 backup, the ffs_mksnap program locked up the entire system.

It wasn't the expected short hiccup that mksnap sometimes causes --  
it was started just prior to lunchtime and when we got back everyone  
had wedged login sessions and IMAP sessions, and ffs_mksnap was in  
"D" state according to ps and idle for about 90 minutes.

The only way out was to reboot the system.

I know this is very un-useful as a bug report, but I'm putting this  
out in case anyone else has noticed this.  It has never happened to  
us before when we were running 5.4 and then 6.1 on this box.  Nothing  
else changed on the system during this time.

The disk in question is on an adaptec RAID controller with the aac  
driver.