add section specifically about adding disks

tsa/howto/ganeti.mdwn

@@ -142,6 +142,14 @@ instance. Then the filesystem needs to be resized inside the VM:
 
     ssh root@test1.torproject.org resize2fs /dev/sda1
 
+### Adding disks
+
+A disk can be added to an instance with the `modify` command as
+well. This, for example, will add a 100GB disk to the `test1` instance
+on teh `vg_ganeti_hdd` volume group, which is "slow" rotating disks:
+
+    gnt-instance modify --disk add:size=100g,vg=vg_ganeti_hdd test1
+
 ### Adding a network interface on the rfc1918 vlan
 
 We have a vlan that some VMs that do not have public addresses sit on.