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I have two unused USB 2 drives. I wanted to stripe them in a RAID0 configuration for fast disk access for virtual machines. (I find running a VMware virtual machine off a USB2 drive to be painfully slow. Especially Windows Vista)

If I have both USB drives attached to the same USB2 hub, will that negate any benefit I gain by creating a RAID0 array? That is to say, is the speed of USB2 the limiting factor or is the speed of the drives? Would I get better performance by attaching one or both drives directly to my computer?

Josh
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3 Answers3

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I would say the only definitive way is to try it and benchmark before and after.

My best hypothesis - for short bursts (small reads and what not), you might see a performance improvement. For sustained transfers, most likely not.

NoCarrier
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The throughput on USB 2.0 is 480Mbps. That is both ways so you can achieve about 240Mbps writing/reading. If you want to take full advantage of RAID be sure to use SATA 3Gbps or SCSI.

What happens is lets say for example you send a 480MB file to the RAID0 array through USB it is cut in half (at least) and the rest is cached so that more data can be sent; the USB Hub processes the data to the HDD's small pieces at a time (<=240Mbps) whereas SATA can process it at <= HALF of SATA 3.0Gbps for example.

This is one reason why you see External HDD's sold with 5400RPM and not 7200RPM (because it's overkill; you will never use the additional 1800RPM because USB is not fast enough yet).

To let someone who may not know. When I say half I am referring to broadband communication. This is because all (modern) media takes into account reading and writing. So you can read at most half the speed of the said speed. Another example is 802.11g 54Mbps TOTAL read AND write... Cut that in half because it's a broadband media and you can at most read and/or write at 27Mbps.

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While I appreciate the technical nature of the earlier answer, I don't know if it addressed the OP question of the hub effect. My understanding of USB devices is that (unlike ethernet) the only communication is from device to computer, so a hub would seem to slow you down in a RAID-like situation. Having said that, there is no guarantee that what appears to be "individual" USB ports on your computer might be "hubbed" inside the case...

P.S. A newer/faster USB 3 standard is emerging in 2011, but the frustration RAID-seekers encounter is that many new machines supporting USB 3 only do so on ONE port! And there are precious few USB 3 hubs...

MarkHu
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