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Hairpin turns

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In an I/O virtualization (IOV)-capable environment, traffic can be sent out of one virtual adapter and into another virtual adapter without regard to the underlying physical hardware. This leads to the very interesting possibility that traffic could be entirely contained in a single physical adapter, which is known as the IOV "hairpin turn." The application for this might be a virtual machine (VM) communicating with another virtual machine through their respective virtual network interface cards (NICs), where the virtual machines reside on the same physical server and the virtual NICs reside on the same physical NIC. In this case, the physical NIC is functioning as a mini-switch. This analogy also works with the top-of-rack IOV units, where the physical adapter is external to the server, but functions in the exact same way.

A storage adapter could be made to operate the same way. Suppose a host server had a Fibre Channel, iSCSI or SAS adapter that was located in a top-of-rack IOV unit. A storage server could be located in the same rack and could, theoretically, use the same storage adapter in the IOV unit as its adapter to the outside world. The physical adapter in the IOV unit would have one virtual adapter configured as the initiator and another virtual adapter configured as the target. Interesting possibilities indeed!

This was first published in February 2010

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