Background: I have a server that has multiple volumes with a total space adding up to nearly 3TB and because of the single extent limit of 2TB for VMFS 3.x, I couldn't store all of the data in VMDKs on one volume. So I had get creative and used Virtual Raw Device Maps for the large disks and mapped them to LUNs on the iSCSI storage.
Challenge: One of the volumes recently ran low on disk space, I have capacity on the SAN; how do I increase the volume size of a vRDM. It's straight forward in a VMDK, edit settings of the VM and increase the size of the disk.
First step is to increase the size of the LUN on your SAN. For me this was easy, open the SAN management software, edit the volume settings, increase the size and click OK.
Next open the vSphere management console, select the VM host > Configuration Tab > Storage Adapters. Click the Rescan All option to rescan the iSCSI bus for the change. Be sure to check the device listing in the lower area to make sure the new size is reflected. We are now ready to work with the VM.
I found KB 1007021 article on VMWare's website and it talks about increasing the size by removing the vRDM and recreating it, shouldn't we be able to do this live.
If you have vMotion you are in luck. If you have only one ESX(i) host, this isn't for you.
Simply perform the same Rescan All function, that is we did above, to the hosts that you can vMotion your VM to. Verify the new LUN size is reflected on that host. Perform a migration of the VM to the new hosts.
Now you should see the new volume size on the guest.
If you are running windows server you can now use either the CLI diskpart in Windows 2003 or the right click / extend volume in Windows 2008 to increase the available disk size.