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Why is USB 3 so slow?

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USB 3.0's SuperSpeed rating is a dismal joke if magazine tests are anything to go by, with transfers at a laggardly 127MB/sec at best, only three to four times faster than good USB 2 products. So why is USB 3.0 so slow?

In theory, USB 2.0 runs at a maximum of 480Mbit/s, whereas USB 3.0 runs up to 4.8Gbit/s: ten times faster. It's generally thought that the fastest deliverable real-world USB 3.0 speed, after allowing for protocol overhead, would be around 400MB/sec. The first magazine benchmarks, of a Buffalo HD-HXU3 and a pre-production Asus device, show that realised real-word transfer rates tend to be a whole lot slower than this. A mere third of it, in fact.

What is the problem?

Either the USB 3 interface is not being run well because the driver is inefficient, or the connected disk drive is simply too slow to fill the USB 3 pipe, or both.

Craig Reid is Buffalo's Product Manager for direct-attached storage in the UK. He points out that the USB controller is actually a complicated piece of kit. It's not a dedicated storage controller like eSATA. The USB controller has to handle up to 127 connected devices, coping with anything from keyboards and mice to printers and webcams, as well as external disk drive products.

That means incoming data traffic from storage devices has to wait while the controller goes through a few hoops before passing it on. Buffalo has a TurboUSB facility which effectively switches off or bypasses some controller complexity and reduces traffic latency through the controller.

Buffalo says its HD-HXU3 external USB 3.0 hard drive offers: "SuperSpeed transfer speeds of up to 130MB/s, 3 times faster than USB 2.0."

Reid is pleased with this 130MB/sec figure, partly because it's faster than eSATA, which, like SATA II, runs at 3Gbit/sec, and because it is a lot faster than USB 2.0 devices. But the HD-HXU3 product can't run at full USB 3.0 speed, Reid says, because: "A single external disk drive cannot saturate the USB 3.0 link."

The disk drive used in Buffalo's HD-HXU3 is a single 7200rpm SATA II drive, with a 3Gbit/s interface - about 40 per cent slower than USB 3.0's 4.8Gbit/s. Buffalo doesn't specify which disk supplier it uses as that can vary with HD-HXU3 capacity level and market conditions.

We know available 7200rpm, 2TB drives come from Hitachi GST and Western Digital. Hitachi GST has two SATA drives offering 2TB: the UltraStar A7K2000 and; the DeskStar 7K2000, both spinning at 7200rpm and using the SATA II 3Gbit/s interface.

Western Digital's 2TB RE4 also spins at 7200rpm and uses SATA II, ditto its 2TB Caviar Black drive. So both the HGST and WD drives can pump data in and out no faster than 3Gbit/s.

Actually Seagate has a 2TB Barracuda XT also spinning at 7200rpm but using the 6Gbit/s SATA interface (SATA III), which is 20 per cent faster than USB 3.0 and could saturate it. Reid said Buffalo does not use this drive, indicating that it is too expensive.

Buffalo will likely go with the 6Gbit/s SATA III interface drives when they are available on the commodity drive market.

Reid said that we need multi-drive storage devices to use USB 3.0 at nearer its maximum speed, suggesting a 4-drive DriveStation Quattro, if it supported USB 3.0, would be able to pump data much faster through it than a single drive product.

Currently, the DriveStation Quattro is a USB 2.0/eSATA device. It transfers data at up to 100MB/sec with that technology. This product looks a prime candidate for a USB 3.0 makeover. ®

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