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Questions about USB 3.0 Speed

Last response: in Storage
August 11, 2011 6:33:30 PM

Hi, I've just installed a USB 3.0 PCI Express Card. I always used USB 2.0. I plugged in my USB 3.0 enclosure and I'm copying movies from my computer to the enclosure. I'm getting speeds of 86 mbps, would that be PCI Express 1.0 or PCI Express 2.0? I read that you get faster speeds with PCI Express 2.0, not so much with 1.0. I have a relatively new Motherboard in my PC, I was just wondering what version of PCI Express that I have. Thanks

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a c 415 G Storage
August 12, 2011 1:56:15 AM

Even the oldest, slowest version of PCIe could transfer data at 250MByte/sec per lane, so the transfer rate you're seeing is nowhere near being limited by PCIe. It's almost certainly the hard drive itself.

Just because USB 3.0 can transfer data at over 600MByte/sec doesn't mean the drive can actually deliver data that fast, any more than a USB 2.0 keyboard can deliver data at 60MByte/sec. For hard drives on USB 3.0, the limitation is in the device, not the connection.

That wasn't true of USB 2.0, since it could only deliver real-world transfer rates of less than 40MByte/sec, and most modern hard drives are faster than that. In that case, the connection WAS the bottleneck.

But even the fastest modern hard drives top out at around 150MByte/sec or so, and that's only for the outermost cylinders. The transfer rate of most drives drops down to only about half as fast when accessing the slower innermost cylinders.
a b G Storage
August 12, 2011 3:42:37 AM

If you want us to tell you what kind of PCI Express you have, you'll need to specify which motherboard you have :) 

Sminial has addressed the rest of your questions.

One point - the transfer rate of USB 2.0 is up to 480 bits/second, and there are 8 bits/byte, but that doesn't mean you'll see a sustained transfer rate of 60 megabytes/second; there's always some overhead. Similarly, the transfer rate of USB 3.0 is up to 4800 bits/second, but you won't see 600 megabytes / second, due to overhead.

Moreover, as Sminial said, magnetic hard drives won't run anywhere near 600 MB/s, especially for sustained transfers. The latest generation of Sandforce SSDs are spec'd to do over 500 MB/s, while some of the other recent releases are spec'd at 400 MB/s.

USB 3.0 is much faster than USB 2.0, and now the slowest part of the data transfer is not the connection.
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August 12, 2011 4:14:50 AM

Hi, I have the MSI 880GM-E35 Socket AM3 880G mATX Motherboard
August 12, 2011 7:09:25 AM

USB 2.0 has raw data rate of 480Mbps, while USB 3.0 I belive is a Gigabyte (3-5 Gbps in my opinion). Wireless-N first transmission speed is typically a standard adapter 150Mbps, so if you do not have to hit near the limit of data transfer. Although USB 2.0 real-world data rate was only 50% of predicted, etc.

USB 3.0 is good for external hard drives and some devices such as a video camera when you need large amounts of data transferred. Otherwise you're not really going to see much more.

Connect a USB 2.0 device to the USB 3.0 port would give you transfer rates of USB 2.0, you will not see any difference. I do not think the iPhone is the USB 3.0 ... it is not as if you could get a USB 3.0 cable, and accelerate the data rate anyway.

Give it a couple of years, and USB 3.0 will be useful when multiple devices are beginning to appear, but for now it's not really a lot of use.