XenServer Creedence Alpha 3, Disk I/O testing (part 2)

We did some more testing of XenServer Creedence Alpha (XSCA3) disk performance, and plotted large streaming reads for a variety of record sizes against both a physical and Brand-X Hypervisor.

Recap:

• System is an AMD6176SE, 2 CPU, 192GB RAM 
• Local storage, 3x 10Krpm SATA, LSI 9261-8i, RAID0, thick provisioned 
• No special settings, tuning, or configuration 
• Testing is with dd and iozone, with and without Direct I/O (dd iflag=direct, iozone -I)
• CentOS 6 2 vCPU, 2GB vRAM (updated 3-07-2014) VM and physical 
• The system was idle 

Physical achieves ~600MB/s transfer speed. 
Brand-X achieves a similar figure.
XSCA3 achieves less than 50% of that, unless Direct I/O is used.
Neither physical nor Brand-X are significantly affected by use (or not) of Direct I/O.

Results for physical without Direct I/O are excluded as with 192GB RAM and only 8GB of test data, transfer rates are in the 2500-1700 MB/s range due to the abundance of RAM for cache. We took no steps to limit the physical CentOS to 2 cores either.
  
We know the disappointing XenServer performance is only for asynchronous (not Direct I/O) disk access, and that the system behaves as expected when running physical or Brand-X hypervisor. The mystery deepens!