LVM Snapshot Question

Hi guys,

thanks again for helping me out.

I am always confused when it comes to creating lvm snapshot. What size should you give to an lvm snapshot ? Should space be available in the volume group in order to create a snapshot? Should the snapshot size be the same as its source lv ?

Lets take this example:

existing lv = lvroot : 10G
VG = vg_sos = 20M free

How will you create a snapshot of the lvroot lvm ?

Thank you

Arrey