How to boot from a slice other than "a"?

[Philip Hallstrom]

>> Filesystem     Size    Used   Avail Capacity  Mounted on
>> /dev/da0s1a    248M    180M     48M    79%    /
>> /dev/da0s1h     14G    496M     13G     4%    /mnt
>> 
>> /dev/da0s1h has an exact copy of /dev/da0s1a using dump/restore.
>> 
>> I want to configure things so that the next time I reboot it will 
>> automatically boot from /dev/da0s1h without me having to do anything (since 
>> it's remote I can't interrupt the boot process).
>> 
>> I know I need to modify fstab on da0s1h.  And if I was on the console I'd 
>> interrupt the boot process and type in 0:da(0,h)/boot/loader, but I'm not.
>
> are you actually interested of running /boot/loader from partition h or 
> mounting root at /dev/da0s1h

Hmm... guess I didn't realize there was a difference :)

I want the OS to "run" from /dev/da0s1h so that I can then completely wipe 
and replace /dev/da0s1a [1].

> if second - add:
>
> vfs.root.mountfrom="ufs:da0s1h"

to which file?

Thanks!
-philip

[1] The box is a new 5.3 install.  My thought was to boot from da0s1h, 
buildworld/kernel for 6 with a DESTDIR pointing to the original / 
(da0s1a). Then reboot and have a clean install of 6.