Boot hangs in single-user mode

[Andrew Hamilton-Wright]

Strangely, it seems that I cannot boot single user, either using "boot -s" from the boot loader, or using the boot menu.  When I get to the point where the root filesystem is mounted, it hangs right after printing the message:
Trying to mount root from ufs:/dev/ada0s1a

Interestingly, there seems to be a bit of a sequence issue, as I have also seen the mount message appear before the audio system comes up, so occasionally, the last item printed is:
pcm0:  <USB audio> on uaudio0

If I boot normally, however, I can consistently reach a login prompt.

I suspect that this may be a race condition of some kind, as yesterday I am sure I successfully booted to single-user while trying to solve a separate problem.


In case the separate problem (failed disk) is relevant, the general situation is this:
- four disk machine:  ada0 (/, /usr, /tmp, /var); ada1 (/research -- data only), ada2 (/home), ada3 (/data -- also data only)
- the disk ada2 has failed
- in preparing to replace ada2, I have commented out all references to it from /etc/fstab

I am rebooting the machine at the moment as I wish to ensure that I know which physical disk is ada2, so want to boot the machine without it plugged in.  I seem to have trouble booting at all with ada2 missing and ada3 still attached, but can boot to multiuser with no problems in either of these two configuration:

- all disks (including the faulty one) plugged in, with ada2 references removed from /etc/fstab
- ada2 and ada3 not physically plugged in, and all references to either removed from /etc/fstab

Neither combination allows me to boot single-user.


While I can clearly go ahead with my disk replacement, this is not only strange and annoying, but potentially problematic.

Has anyone else seen anything like this?  I notice that there are several messages (dating back to 2004) in the list indicating 'hang after "Trying to mount root"' or 'hang after sbin_init' (which is the message that will be seen when booting single-user in "verbose mode").

Thoughts?  Ideas?

Thanks,
Andrew.