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The problem lies in that W2K8 requires SCSI-3 disk reservations and for the most part VMs only support the SCSI-2 disk protocol. The exception would be RDMs in physical mode so today that would be your only option besides using an iSCSI initiator in the VM? Have you tested this with MS Virtual Server to see if the Adaptec 7870 SCSI adapter based SCSI controllers will support SCSI-3 disk reservations? I haven't heard of any plans on VMware's part to change the VMs to use a SAS virtual disk.
- Persistent Reservations - WIndows Server 2008 Failover Clustering supports the use of persistent reservations. This means that directly attached SCSI storage will no longer be supported in 2008 for Failover Clustering. Serially Attached Storage (SAS), Fiber Channel, and iSCSI will be the only supported technology. However, not all vendors support persistent reservations, so this will be a problem as organizations move to Windows Server 2008 Failover Clustering without full and proper testing.
I'm having the same trouble but I'm using an RDM with both adapters set to physical sharing mode. The config I'm using worked great for 2k3.
Does this mean that my SAN doesn't support SCSI-3 reservations?
I think the problem is that if you add a virtual disk to a guest it'll be a SCSI disk, regardless of whether it's an RDM or not, or what your actual storage backend is.
As Dave.Mishchenko posted above, direct attached SCSI is no longer supported by Server 2008 clustering. You have to use SAS, FC, or iSCSI. The only option within ESX then is to use an iSCSI initiator on the guest. I haven't seen anything to suggest VMware will be adding SAS virtual disk support for guests anytime soon.