Virtualization Pro Tips Blog

iSCSI and Hyper-V: Connecting from the VM, or From the Host?

Ben Armstrong posted an excellent article at the Virtual PC Guy’s Blog just before the holiday.  There, he discussed why it is a better idea to initiate iSCSI connections from the Hyper-V host rather than inside the virtual machine.

Because Hyper-V has the same operating system for host and guest, both halves have the ability to use their own initiators for connecting to iSCSI storage.  A virtual machine can connect directly to storage through its iSCSI initiator.  Or, the host can connect to that same storage via its iSCSI initiator.  Once connected, the host can then present that storage to a VM as a virtual hard disk or pass-through disk.

At first blush, these might seem like equivalent ways of getting to the same end – essentially, more storage for a VM.  Not true.  With this decision, the end doesn’t necessarily justify the means.

When possible, you should use the iSCSI initiator on the host to connect to storage rather than directly inside a virtual machine.  One reason for this is that the host’s network stack can enjoy a greater set of network offload and acceleration technologies than can the virtual machine.  Armstrong puts it best when he states, “…a software iSCSI initiator in the virtual machine will use a lot more CPU power to achieve the same results as a hardware (or software) iSCSI initiator in the management operating system.”

There is one important caveat:  iSCSI storage initiated inside the VM is required for guest clustering, where the VM itself is a member of a Windows Failover Cluster.


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