I TAKE NO RESPONSIBILITY FOR LOST DATA!!! BACKUP IS YOUR FRIEND!

This is a quick and dirty guide to span an existing /home partion over several discs using only lvm2

Example:
sda1 is /boot
sda2 is /
sda3 is /home

sdb is empty

------------------------
Load the needed kernel module
#  modprobe dm-mod


First create a partion with fdisk/parted/something on sdb that fills the entire disk using your favorite partion tool


Then, set up lvm2 on it:
#  pvcreate /dev/sdb1


Create a volume group, and add sdb1 to it:
#  vgcreate vg1 /dev/sdb1   #vg1 can be any name


Create a new logical volume (partion) on the volume group (using all the space):
#  vgdisplay vg1 | grep "Total PE"
Total PE              10230  #ofcause the actual number will be different
#  lvcreate -l 10230 vg1 -n home
      

Activate the volume group:
#  vgchange -a y vg1


Create a filesystem on the logical volume:
#  mkfs.ext3 -O dir_index /dev/vg1/home  


Create a temporary folder and mount the new partion:
#  mkdir /mnt/tmp_home
#  mount /dev/vg1/home /mnt/tmp_home  


Move over the data from the old /home
#  cp -a /home/* /mnt/tmp_home


unmount the old /home
#  umount /home


Set up lvm2 on the old /home
#  pvcreate /dev/sda3


And add it to the volume group
#  vgextend vg1 /dev/sda3


To add space to the logical volume (partion)
#  lvextend -L +50G /dev/vg1/home  #50G can be any number of gigabytes, you can also use M for mega. 



To add all the free space on the volume group to the logical volume:
# vgdisplay vg1 | grep "Free PE"  #not on a lvm right now, but I think it's "Free PE", look at the output from vgdispal if I'm wrong
Free PE              2000
#  lvextend -l 2000 /dev/vg1/home


Unmount the filesystem:
#  umount /mnt/tmp_home


Grow the filesystem:
#  resize2fs /dev/vg1/home


Mount the new home to /home
#  mount /dev/vg1/home /home


Edit your fstab:
#  vim /etc/fstab
Change /dev/sda3 to /dev/vg1/home


Make arch load the kernel modules at bootup: (not realy needed since you don't have lvm2 on /, but still a good thing to do)
#  vim /etc/mkinitcpio.conf
Add lvm2 to the HOOKS aray


Run mkinitcpio
#  mkinitcpio -g /boot/kernel26.img  #i asume that you are running the latest kernel, so no need for -k and -c


Edit rc.conf to make arch activate the volume group at bootup:
#  vim /etc/rc.conf
Change USELVM= to "yes"


That shoud be it.