HotAdd and SCSI Controller IDs

When using HotAdd backup, always add SCSI controllers to virtual machines in numeric order.

Most systems lack an interface to report which SCSI controller is assigned to which bus ID. HotAdd assumes that the unique ID for a SCSI controller corresponds to its bus ID. This assumption could be false. For instance, if the first SCSI controller on a VM is assigned to bus ID 0, but you add a SCSI controller and assign it to bus ID 3, HotAdd transport may fail because it expects unique ID 1. To avoid problems, when adding SCSI controllers to a VM, the bus assignment for the controller must be the next available bus number in sequence.

Also note that VMware implicitly adds a SCSI controller to a VM if a bus:disk assignment for a newly created virtual disk refers to a controller that does not yet exist. For instance, if disks 0:0 and 0:1 are already in place, adding disk 1:0 is fine, but adding disk 3:0 breaks the bus ID sequence, implicitly creating out-of-sequence SCSI controller 3. To avoid HotAdd problems, you should add virtual disks in numeric sequence.

To deal with more disks than can fit on a single controller, you must add some permanent dummy disks to the proxy VM, one on each additional controller that might be needed. Adding only the controller does not cause the controller to remain attached to the proxy VM. A real VMDK must be added on the controller to keep it attached to the proxy VM.