Thanks to Ash for pointing me at this: http://www.vmware.com/files/pdf/vsphere_perf_exchange-storage-protocols.pdf
Microsoft Exchange Server 2007 is a storage intensive workload used by a large number of IT organizations to provide email for their users. In this paper, a large installation of 16,000 Exchange users was configured across eight virtual machines (VMs) on a single VMware vSphere 4 server. The performance of this configuration was measured when using storage supporting Fibre Channel, iSCSI, and NFS storage protocols. The results show that each protocol achieved great performance with Fibre Channel leading the way, with iSCSI and NFS following closely behind.
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