Is an external USB3 disk really almost as fast as an internal SATA disk?

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?

Sandra

Posted 2012-07-11T23:45:04.927

Reputation: 1 893

45Gbit/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 – 2012-07-11T23:55:44.533

So from the users point of view, the external USB3 disk is just as fast as the internal SATA disk? – Sandra – 2012-07-12T00:04:51.920

3My 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 – 2012-07-12T00:09:49.700

Unattractive for what purpose? – Andrew – 2012-07-12T02:11:17.770

@Zoredache, not true for high end SSDs – soandos – 2012-07-12T04:28:12.457

Answers

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.

wfaulk

Posted 2012-07-11T23:45:04.927

Reputation: 5 692

What about SSDs? – soandos – 2012-07-12T04:28:58.400

1@soandos: SSDs can saturate both interfaces. In such a case, SATA 6.0 Gb/s will outperform USB 3.0. – bwDraco – 2012-07-12T04:35:02.013

@DragonLord, this was more of a prompt for wfaulk, but yes, I believe that is the only case where it makes a difference – soandos – 2012-07-12T04:39:40.727