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I have a Kingston V200 256GB SSD. When connected to my SATA 6.0gbps port on my motherboard I'm seeing sequential read speeds of 300 megabytes / second.

When I put the drive into a sata to USB 3.0 enclosure and connect it to my USB 3.0 motherboard port my sequential read speed is maximum of 200 megabytes / second and average of 120 megabytes / second. I tried the enclosure on another machine and got 120 megabytes / second as well.

I'm guessing the enclosure itself is the bottleneck. Anyone know of reviews/benchmarks on enclosure chipsets themselves so I can buy something that isn't going to slow me down?

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200MB/s is 1.6Gbps. Are you realistically being slowed down by this, or do you just want to be as fast as SATA? USB 3 has a theoretical maximum of 5Gbps but realistically, what you're seeing isn't that bad. –  user3463 Jul 14 '12 at 19:28
    
Yet other benchmarks I've seen show USB 3.0 external SSD drives getting 300MB+. So if my SSD can get 300MB/s (2.4gbps) when connected to SATA it should be the same speed on USB 3.0. So the SSD is capable of 2.4gbps which is well within the range of expected real-world USB 3.0 bandwidth. I think my issue is a crappy USB 3 enclosure. –  rbeede Jul 14 '12 at 23:34
    
Maybe, and maybe it’s your cables, your motherboard’s USB 3.0 implementation, your USB port… Have you tried using a high-quality cable, connected to a USB port directly on the motherboard? –  Synetech Jul 22 '12 at 2:18
    
Yes, I tried all of those things. I reviewed the motherboard benchmarks and others have gotten better performance. –  rbeede Jul 23 '12 at 14:17

2 Answers 2

USB is slower then SATA. The new SATA 3 standard operates at a theoretical maximum rate of 6.0 Gbps, while USB 3 operates at a theoretical maximum of 4.8 Gbps bi-directional. If we crunch these numbers...

      USB 3.0                    SATA 3
      4800Mbps            6000Mbps each way
      600 MBps            750 MBps each way

      USB 2.0                    SATA 2
480Mbps divided by 2      3000Mbps each way
  240Mbps or 30MBps       375 MBps each way

EDIT: As per @MrAlpha, I stand corrected on USB 3; it can take advantage of Full Duplex (as described here)

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+1 Nice answer. –  user3463 Jul 14 '12 at 19:48
    
Merci, @RandolphWest –  Canadian Luke Jul 14 '12 at 19:50
2  
-1 This is not correct. USB 2.0 is half duplex, meaning that it can do full 480Mbps (not accounting for encoding) in either direction. It just can't do it in both directions at the same time. USB 3.0 support full duplex, same as SATA so it can do full speed in both directions. Also, the actual speed of SATA 3 once you account for 8b/10b encoding is and the prefix mess is ~572MiBps. This compares to USB 3.0s ~458MiBps. –  Mr Alpha Jul 14 '12 at 20:03
    
@MrAlpha I am not seeing any resources on the Internet that support this argument. Can you please provide a link so I may update my answer? –  Canadian Luke Jul 14 '12 at 21:10
    
@MrAlpha Nevermind, found a source –  Canadian Luke Jul 14 '12 at 21:13
up vote 1 down vote accepted

http://electronicdesign.com/article/embedded/whats-difference-usb-uasp-bot-73593

The enclosure lacks the faster UASP mode. Real world throughput of USB 3.0 has a maximum of 400MB/sec (USB 3.0 has 36% overhead in the best case scenario).

Typical USB 3.0 devices seem to be only BOTS enabled limiting them to a maximum of 250MB/sec although in my case this seems to be a slower drive enclosure.

eSata 3.0gbps is still faster than USB 3.0 in the average case at 300MB/sec real world. Disappointing.

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