Doing some speed tests of IP based storage gave me some good results. First I enabled Jumbo frames on my dedicated IP storage Ethernet card, then I set the MTU on the vSwitch and the VMKnic to 9000 (Scott Lowe has a great write-up on doing this on his website). I then mounted a NFS volume as Datastore and did a copy of a VMDK file which currently resides on a 7200rpm SATA 3 drive to the mounted NFS volume.
I copied the 8GB file in under 2 minutes and my average MbTX/s was around 525Mb (65MBps) – keep in mind, this was a copy from a local SATA disk (low IOPS) and I was able to get around 65MBps — not bad!
The biggest thing to remember, VMDK access needs low latency…not high throughput — this is why NFS has become so popular for VMDK storage.