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I've previously read suggestions for running a virtual machine from an external hard drive to increase performance. With this in mind, I'm curious what the introduction of a new technology like Thunderbolt will do to this.

I know that it can't hurt performance, but I'm curious about this -- as slow as hard disk drives are (perhaps ~100 or 200 MBps?), how noticeable of a performance difference could there be for operating a virtual machine? If USB (even 2.0) transfers faster than the disk can transfer, isn't the bottleneck already the hard drive?

Note: I know that solid-state drives introduce an entirely new situation; I'm not referring to them.

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up vote 2 down vote accepted

Hard disks have best case sustained transfer rates in the 100M byte per second ballpark. The best SSDs are in the 500Mbyte/s range (last time I looked).

USB 2.0 specs at a theoretical 480M bit per second. Serial schemes usually do some kind of bit-to-byte encoding, like 8b/10b or NRZI with bit-stuffing. So to convert the speeds, you can make it easy on yourself and just divide bits by 10 to get bytes.

USB's real-world performance is well below theoretical. Some hard disks can definitely saturate USB 2 (at least some of the time). Some SSDs can probably saturate USB 3. Thunderbolt is at least twice as fast as USB 3, and does not suffer from host overhead; it currently has headroom for single drives of either variety.

Is the difference noticeable? If you have a fast enough hard drive and run it on USB 2, you should be able to tell if you switch to Thunderbolt -- for some workloads.

As for virtualization, no specific extra advantages come to mind, although improvements like VT-d or its successors might work better with Thunderbolt.

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Whatever the slowest device is in a chain of devices involved in transferring data, that is your bottleneck.

Intel's Thunderbolt technology promises 10 Gbps transfer rates:

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Since it is basically a external PCIe bus with multiple channels full duplex and chaining capability, I would say hell yea. Hook a 24 bay external raid enclosure without having to pay a 10gbps ethernet of fiber channel tax is going to make a big difference.

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