swapping LVs about

[Gabriel M Dragffy]

Hi,

How is it possible to, for example, take a snapshot of root and then
mount that snapshot in place of the real root partition, to try out some
ideas or changes? Of course when I try to unmount the root partition I
can't! Then after this how is it possible to swap them back to get the
original root partition back?

I also have something that I don't understand. My root partition is 300M
and has used about 170M of space. I created a 100M snapshot volume of it
and it worked - how come it had enough space? How much space does a
snapshot actually need, or more simply how does the snapshot work? I
have read that excellent LVM howto, but these two points I don't fully
understand and I don't think they are covered in more detail in the
howto.
Thanks a lot.
Gabe