Do you mean standard Xen VBDs or is there an iSCSI thing called "VBDs"? If you mean what I think you mean, I'm not sure it would currently work: Xen's / XenoLinux's VBD code AFAIK are purely capable of virtualising local physical disks.

To clarify what I mean (apologies if this is redundant info) - VBDs are implemented at the Xen level, not at the XenoLinux level: when you create a VBD in dom0 for a guest domain, dom0 is telling Xen "grant this domain access to this bit of disk". The XenoLinux VBD driver then talks to Xen (not dom0) to access the VBD that have been created (the special case is that Dom0 has a vbd for each physical disk, giving the impression of a normal setup).

Xen's VBDs can't re-export network-based block devices from dom0 (or indeed any other kind of device that indirects through network or filesystem layers in dom0), since xen is not aware of these higher layers.

To re-export iSCSI drives from dom0, I think you'd currently need to use NFS or something similar - to re-export them appearing as "just another VBD" to the guest would require extra code.

Is this relevant or do I have the wrong end of the stick?

Mark