I understand that VMs can not be vmotioned to other hosts unless they exist on the same LUN.....but, since the hosts don't reside on LUNs I don't understand this concept. When I want to vmotion a VM it is going from one host to another....not LUNs. What am I missing?
Vmotion requires shared storage that all ESX servers can see (SAN, iSCSI, NFS). If you use local disk storage you cannot use Vmotion. So when you Vmotion a VM from one server to another none of the files are actually being moved from one ESX host to another the VM is simply being re-located in memory from one ESX host to another.
Vmotion requirements - http://www.vmware.com/pdf/vi3_admin_guide.pdf (Chapter 14)
LUNs are shared between ESX hosts. During vmotion only active RAM of the vm is copied to the target host. When RAM is in sync IO (and all cpu) activity is done by the new esx host still to the same lun.
Basically vMotion transfers the VMs memory and CPU state from one ESX to another.
Disk files are (normally) too big to transfer them to another host without interrupting the VM - which would make vMotion useless - therefore
the need for a shared storage.
thanks very much to everyone. So, I understand now. As long as all the Hosts have access to the same Lun or Lun's, moving VMs with their files located on a shared LUN can be moved between the hosts since the VM files remain in the same LUN location. The VM is hosted on a different Virtual host server but its files are still in the same location.