A: A recent comparison test by Demartek.com, announced at TechEd North America in my session "iSCSI and Windows Server: Getting Best Performance, High-Availability, and Better Virtualization," found that iSCSIdDirect connections—those that are directly attached to a Hyper-V virtual machine, not attached as pass-through disks—experience a slight degradation in IOPS, database read average latency, and log write average latency.

The test compared the two connection approaches against an Exchange database that was stressed using the Exchange Jetstress 2010 tool. It was set to 1500 mailboxes of 750MB each, two threads, and Exchange IOPS setting at 0.18. The results of this test are listed below:

Native

iSCSI Direct

Achieved IOPS

519.816

464.12

Database Read Average Latency

9.459 msec.

11.909 msec.

Log Write Average Latency

8.236 msec.

9.732 msec.

You can watch this Tech Ed session, which includes additional information about the test here.