Simple way to clear arbitrary drive metadata?

[Warren Block]

On Mon, 1 Oct 2012, Konstantin Kukushkin wrote:

>> gmirror is a problem.  If the gmirror kernel module is loaded, drives
>> with gmirror metadata create a mirror.  GEOM prevents writes to the
>> drive then.  sysctl kern.geom.debugflags=16 allows writes, but the
>> mirror is still in memory and running.  'gmirror stop' (which the system
>> also does on shutdown) helpfully writes the whole metadata block back to
>> the drive.  After reboot, it's right back where it was.
>> 
>> Seems like the only way to deal with gmirror is to have the user check
>> for it directly, and stop the mirror if the attached drive is a member.
>
> You may use 'geom <class> clear' command to clear metadata from 
> disk/partition/other GEOM provider. This will work only on stopped mirrors or 
> while geom_mirror not loaded, of course.
> For gmirror you can 'remove' component from running mirror (even if the 
> component is last). Metadata will cleared after removing, so gmirror remove 
> <mirror_name> <component_name>' is very usable.

Yes, that's what I used.  This was initially for an update to the 
gmirror section of the GEOM chapter of the Handbook:
http://www.wonkity.com/~wblock/gmirror/geom-mirror.html

The problem with gmirror remove or clear is that the user must check to 
see whether the drive is part of an active mirror to know which command 
to use.  Seems like "remove" should not require the mirror name and 
would work whether the mirror was active or not.

> You can determine what GEOMs metadata living on disk $disk using command 
> 'geom <class> dump', like this:
> for class in mirror stripe concat raid3; do
>  geom $class dump $disk
> done

Is there a utility that can do that in the other direction?  Given a 
disk, identify the type of metadata on it?