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According to the specs of USB3 its max speed is 5Gbit/s and SATA is max 6Gbit/s.

Can it really be correct that SATA is only 20% faster than an external USB3 disk?

Or are there some latencies that kills the USB3, and therefore make it unattractive?

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migrated from serverfault.com Jul 12 '12 at 4:24

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5Gbit/s is the speed of a wire, not the speed of a disk. It seems unlikely that you could saturate the link speed of the USB3, or SATA cable with a single disk. –  Zoredache Jul 11 '12 at 23:55
    
So from the users point of view, the external USB3 disk is just as fast as the internal SATA disk? –  Sandra Jul 12 '12 at 0:04
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My guess is that you should see no difference. USB3 is probably going to have more overhead since it needs to support devices other thing storage. Also consider that aside from flash drives almost no hard disks actually have a USB connection on them. A typical USB hard disk is a SATA disk with a USB-SATA converter in it. That converter could add additional latency and slow things down. So I suspect the answer is that it depends on how well the USB to SATA converter works. –  Zoredache Jul 12 '12 at 0:09
    
Unattractive for what purpose? –  Andrew Jul 12 '12 at 2:11
    
@Zoredache, not true for high end SSDs –  soandos Jul 12 '12 at 4:28

1 Answer 1

up vote 6 down vote accepted

From Wikipedia:

The (USB3) "SuperSpeed" bus provides a transfer mode at 5.0 Gbit/s additionally to the three existing transfer modes. The raw throughput is 4 Gbit/s, and the specification considers it reasonable to achieve 3.2 Gbit/s (0.4 GB/s or 400 MB/s) or more.

Even that, though, is far faster than what a single drive is likely to be able to manage.

USB3 does also move from half-duplex to full-duplex communication, so that should improve latency significantly over USB2.

My guess is that if you're talking about a single drive on each bus, you wouldn't see a lot of difference between them. But that's just theoretical and based on my limited understanding of USB.

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What about SSDs? –  soandos Jul 12 '12 at 4:28
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@soandos: SSDs can saturate both interfaces. In such a case, SATA 6.0 Gb/s will outperform USB 3.0. –  DragonLord Jul 12 '12 at 4:35
    
@DragonLord, this was more of a prompt for wfaulk, but yes, I believe that is the only case where it makes a difference –  soandos Jul 12 '12 at 4:39

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