shrink lvm to get spare space

[Zhengquan Zhang <zhang.zhengquan@gmail.com>]

Dear debian community,

on on of my systems I have

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/debian-root
                      224G   12G  200G   6% /
tmpfs                1008M  8.0K 1008M   1% /lib/init/rw
udev                   10M   80K   10M   1% /dev
tmpfs                1008M     0 1008M   0% /dev/shm
/dev/sda1             228M   28M  189M  13% /boot

Now I would like to shrink / by 30G so that I have 30G for another OS. I
wonder if this kind of shrinking is possible. I am using ext3.

Thanks for any pointers!

Regards,

-- 
Zhengquan