Snapshot deadlock while dumping

[Ulrich Spoerlein]

Hi,

One of our fileservers deadlocked, again. It is running RELENG_6 from
2006-11-14 and was running dump(8) -L on a 11% filled 400GB UFS2
volume. It is hanging for 3h hours now, and there is no disk activity.

# ps axl | grep snap
    0    46     0   1  -4  0     0     8 snaplk DL    ??   98:58.88 [bufdaemon]
    0    48     0   0  -4  0     0     8 snaplk DL    ??   68:22.58 [syncer]
    0 15179 11192   5   8  0  1708  1044 wait   I+    p1    0:00.00 sh
-c /sbin/mksnap_ffs /export/
    0 18738 15179   0  -8  0  2776  1756 getbuf D+    p1    0:04.07
/sbin/mksnap_ffs /export/homes

Quotas are enabled in the server, but the filesystems are currently
mounted without quota support (they were once mounted with userquota,
though).

Thanks,
Uli

PS: I can't break to DDB, as it is not configured for this server.
What are the recommended DDB settings for _production_ servers? I want
them to reboot on panic, but be able to grab the panic string via
serial console. Is something like this gonna do the trick? Is there
some kind of performance impact?

options KDB
options DDB
options KDB_UNATTENDED
options ALT_BREAK_TO_DEBUGGER

It should *NOT* enter the debugger, if I plug/pull an RS232 cable. I
read somewhere, that some controllers do send a break if the cable
gets pulled, IIRC.