Can GEOM be used to intercept the I/o calls to an existing mounted device?

[Tapan Chaudhari]

Hi All,
    I am a newbie to FreeBSD and GEOM. I wanted to intercept the i/o calls
going to a particular mounted device by writing some driver.
I came across  geom and thought it might work. I tried to create a new
device, mount it and then tried using gmirror to mirror the device :-

#dd if=/dev/zero of=file1 bs=1M count=10
#dd if=/dev/zero of=file2 bs=1M count=10
#mdconfig -f file1
#mdconfig -f file2
#fdisk -B /dev/mirror/md0
#newfs /dev/mirror/md0s1
#mkdir /mnt/mirror0
#mount /dev/mirror/md0s1 /mnt/mirror0/

#gmirror label -v -n -b round-robin gm0 /dev/md0
gmirror: Can't store metadata on /dev/md0: Operation not permitted.

It gave me this error "gmirror: Can't store metadata on /dev/md0: Operation
not permitted."

Am I doing something wrong? Or GEOM does not permit me intercept the i/o
calls? If not geom, is there any other mechanism by which I can achieve it?


Thanks,
--Tapan.