These undocumented options are there to support growfs in Solstice Disk Suite. And yes, you can use them directly (at your own risk) - this
 has been discussed on comp.unix.solaris before. I've used them to expand a mounted filing system (although it wasn't the root) while the
 system was running. You want
 
   /usr/lib/fs/ufs/mkfs -G -M / /dev/rdsk/c0t0d0s0 [number-of-sectors]
 
 having made sure that c0t0d0s0 has been expanded to at least that size first... and that it doesn't overlap any other slice being used for a
 filing system or swapping, of course! The size is the new total size; all other mkfs parameters are taken from the filing system being
 expanded.
 
 To expand a non-mounted filing system (I've done this even more often) just use -G without the "-M /where/its/mounted".
 
 Doing all this via newfs would be mighty strange, and I suspect wouldn't work!
 
 Chris Thompson
 Email: cet1 [at] cam.ac.uk

Expanding Solaris filesystems
By : anonymous ( Thu May 5 07:28:55 2005 )

extend a veritas file system while mounted
By : anonymous ( Thu Jan 23 12:39:07 2003 )

UFS grow
By : thorstenb ( Sat Dec 14 17:27:39 2002 )