We wanted to expand an iSCSI volume on the Compellent SAN and in turn expand the datastore in Ovirt. This feature has gone from being unsupported to supported (with manual intervention) in Ovirt 3.4 and above. It will at somepoint become a fully automated process.

1) Expand the volume on the Compellent. In the GUI right click the volume --> Expand

2) In Ovirt Admin Portal put the storage domain into maintenance mode.
Ovirt Admin Portal --> Data Centers --> Select the DC --> In the lower panel select storage --> Maintenance
When complete you should see a spanner icon next to the storage domain.

3) In the Ovirt Admin Portal --> Hosts tab Identify the node that is the SPM manager and start the next steps on that host first.

4) Verify the current disk size in multipath:
 multipath -ll | grep -A 11 36000d310004edc00000000000000002d
In this case the multipath volume shows the old size (21G). We have expanded the volume to 25G.

5) Rescan the iscsi interfaces to resize the underlying LUN:
[root@primitivo ~]# iscsiadm -m node -R -I iface0
[root@primitivo ~]# iscsiadm -m node -R -I iface1

6) Now we can check that the underlying LUN's have been expanded.
Take the devices from the multipath volume and check the size:
[root@primitivo ~]# fdisk -l | grep /dev/sdbf
We can see the underlying LUN shows the expanded size of 26G. Repeat this for the other devices (sdbi, sdbg, etc).

7) Resize the multipath volume:
[root@primitivo ~]# multipathd -k"resize map 36000d310004edc00000000000000002d"
You should now be able to verify the resize with multipath -ll:
[root@primitivo ~]# multipath -ll
Volume now shows the new size of 25G.

8) Resize the LVM volume. The physical volume name should match that from the mulitpath name:
Start with the physical volume:
[root@primitivo ~]# pvresize --setphysicalvolumesize 25G /dev/mapper/36000d310004edc00000000000000002d
This should come back saying "1 physical volume resized".
Now scan the volume groups:
[root@primitivo ~]# vgs

9) Now repeat steps 4,5,6 and 7 for the remaining nodes in the cluster. Once all nodes have been completed take the volume out of maintenance:
Ovirt Admin Portal --> Data Centers --> Select the DC --> In the lower panel select storage --> Activate
Once the domain comes back online it should show the new size. Guest disks can then be resized if required.

BUUUUT

very last step, no more disk to resize... :/