“The purpose of his test was to find the best performing iSCSI target with all other things being equal. Under this criteria alone, it would be a split decision between Openfiler and Microsoft.
However, the Microsoft solution requires the purchase of a licensed copy of Windows 2008 R2 server to run its filer software so it isn’t exactly free. In addition, Openfiler can be run from a USB key, leaving all disks available for storage, where Microsoft cannot easily be run from anything other than a hard drive, taking up storage space for the OS.
With this added criteria, Openfiler emerges as the iSCSI champ.”
Have a look at the results here (warning: loads of graphs and screenshot): FreeNAS / Openfiler / Microsoft iSCSI performance shootout.