The terminology of LVM:
physical volumes – physical disks and disk partitions to be used by LVM
volume groups – virtual disk composed of physical volumes
logical volumes – virtual partitions which we use and could be easily managed
Prepare the partitions and disks:
pvcreate /dev/hda4
pvcreate /dev/hdb
Create volume group of name mygroup-vol:
vgcreate mygroup-vol /dev/hda4 /dev/hdb
You should be able to see your new volume group in the vgscan output:
# vgscan
Reading all physical volumes. This may take a while...
Found volume group "mygroup-vol" using metadata type lvm2
Now you can create actual partitions with lvcreate, view them with lvdisplay and manage them with these and other LVM tools (their names start with lv prefix). Logical volumes can be accesed as regular devices in /dev/mygroup-vol/.
lvcreate -n test --size 1g mygroup-vol
More information in lvm(8) manpage.