Same HDD is slower in a TB2 enclosure than a USB 3.1 enclosure...makes sense?

[camner]

I have a late 2015 27" iMace with a USB 3.1 OWC dual dock and an OWC Thunderbay (TB2) four-bay enclosure. I took the same HDD and ran speed tests with each enclosure.

I was surprised to see that the read and write speeds were about 30% slower in the Thunderbay than in the USB 3.1 dock. Given that TB2 has higher bandwidth than USB 3.1, I don't understand why that should be the case. In each case, the HDD should be the bottleneck, not the enclosure, so I would have expected the two enclosures to turn in about the same performance.

Any ideas on why this may be the case?

 

[joevt]

Thunderbolt has more bandwidth, but adds latency. The bandwidth is limited by the HDD so what you're seeing might just be latency.

Mac -> USB controller -> USB to SATA adapter, HDD
Mac -> Thunderbolt -> SATA controller -> HDD.

Therefore, you want to use higher bandwidth devices (exceed the ~430 MB/s of USB 3.0) to take advantage of Thunderbolt (~1500 MB/s)

A different example: a Thunderbolt eGPU has more bandwidth (22 Gbps) than a PCIe 3.0 x1 GPU (8 Gbps) but the latency may make it have lower fps. The eGPU connected with Thunderbolt has a problem because the graphics driver is usually not using all the extra bandwidth of Thunderbolt compared to the GPU using a direct PCIe 3.0 x1 connection. To take better advantage of an eGPU, you want to use higher resolution / higher quality settings (make the GPU work longer on a frame instead of trying to send more frames).

 

[Fishrrman]

I'm not surprised.
I've seen a number of posts from folks in the forum, who used thunderbolt drives because they believed "they were faster", and were actually getting LOWER read speeds than they would get from ordinary USB3...

 

[camner]

Thanks both for your replies, @joevt and @Fishrrman!

So the bragging about Thunderbolt speeds is mostly marketing hype? Are the circumstances under which an external drive connected via TB3 (or 2) would read/write faster than USB 3?

What about connecting an external USB enclosure via a TB dock? I presume that would also suffer from the same latency issues?

 

[joevt]

So the bragging about Thunderbolt speeds is mostly marketing hype? Are the circumstances under which an external drive connected via TB3 (or 2) would read/write faster than USB 3?

Thunderbolt is faster for large sequential read/writes - if you have a RAID or SSD or NVMe that can go faster than 500 MB/s.

What about connecting an external USB enclosure via a TB dock? I presume that would also suffer from the same latency issues?

The USB would have to go through Thunderbolt so it would be slower than your Thunderbay 4 test - unless we're wrong and there's something actually wrong with the Thunderbay 4.