Introduction to VMDirectPath
Although hardware independence is fundamental to many of the advantages of virtualization it also creates a problem when you need to connect a specific hardware device to a virtual machine. Although SCSI passthrough has been available in ESX for some time accessing other types of device, especially USB, has always required alternative solutions such as USB over IP hubs. With the release of vSphere4 VMware have introduced a new feature called VMDirectPath I/O which allows up to two PCI(e) devices on the host server to be connected to a Virtual Machine. Officially this is provided to reduce the latency and hence improve the performance of devices such as 10Gb NICs and Host Bus Adapters, in fact only a handful of such devices are supported by VMware. However in fact pretty much any PCI(e) device can be connected, just bear in mind that VMware will not help with any problems you may encounter.
So far, so good, but there are a number of things to consider before you can go ahead and connect a device, first of all the system requirements. VMDirectPath requires a host server which supports Intel's VT-d or AMD's IOMMU technology, which means the latest generation of chipsets so if your server is over a year old you're probably out of luck. The second significant issue is that because you are directly connecting your virtual machine to physical hardware several key features will no longer be available on that VM, namely:
- vMotion and Storage vMotion
- Fault Tolerance
- Snapshots and VM suspend
- Device Hot Add
Provided that hasn't put you off we'll now look at how to do it, in this example mapping the USB2 root hub on an HP DL380 G6 server so an external USB drive can be connected to an XP VM.
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