labelling root file system (RELENG_8)

[Matthias Andree]

Greetings,

I've tried to re-label an existing (and up to date) 8-STABLE
installation's root partition.  This failed, tunefs reports it cannot
write the super block.

I have attempted this sequence:

1. reboot (through BIOS and loader) directly into single-user mode
(boot -s)
2. sysctl kern.geom.debugflags=16
3. tunefs -L root /dev/ada0s4a    # that's the dev I mount the root
                                  # partition from

Still no joy => tunefs cannot update the super block.

Remounting / as rw doesn't help, as expected.

The root fs uses softupdates but no journalling -- what other hoops do I
need to jump through to create labels for my root ufs file system?

Thanks.

Best regards,
Matthias