Last Week on My Mac: Complexities of Thunderbolt hubs

I seldom stick my neck out and make predictions, but if I was looking to sell any Mac peripheral it would be a Thunderbolt 4 hub. Just as with USB-A in the past, no sooner are you marvelling at how many ports your Mac has got, than you’re wondering how to connect all your peripherals. Docks are expensive, bulky and inflexible: give me a compact hub and a couple of cheap adaptors to chain other devices from it, and I’ve got more flexibility, lower cost and better performance.

Last week I started to focus on the last of those, just how well can Thunderbolt 4 hubs work with some premium high-speed SSDs? The answer isn’t as clear as I had hoped, and gets murkier by the day. In two cases, when an SSD normally capable of writing at 2.2 GB/s was connected through a hub and written to at the same time as another faster SSD, its write speed collapsed to 800 MB/s, the same as an SSD of half the cost. Even when writing to that SSD alone the Mac only managed 1.5 GB/s, the same as many older and cheaper Thunderbolt 3 SSDs. How could that be?

The answer could lie anywhere from macOS to Thunderbolt controllers in hubs and SSDs. By now we should be used to Thunderbolt not quite delivering what we expect: the best examples are all those certified SSDs that can’t read or write any faster than 1.5 GB/s. That’s because Thunderbolt 3 includes ‘Low Power’ controllers (notably Intel’s JHL6240) that only use two of the four PCIe 3.0 lanes available. As each lane delivers up to 10 Gb/s, those SSDs we thought complied with a 40 Gb/s standard were actually only halfway there with 20 Gb/s.

There’s also the problem of using DisplayPort video over Thunderbolt, in combination with data devices like SSDs. Controllers give priority to the needs of video over data, so driving a large display will result in poorer SSD performance. That’s particularly significant for those connecting both displays and SSDs to their hubs, although my previous measurements with an Apple Studio Display showed little reduction in SSD benchmarks.

If those might discourage you from considering a hub, its performance during combined reading and writing appears to exceed what you’d expect through its single Thunderbolt connection with the Mac.

In my latest tests, when the fastest of three SSDs was reading, and the slower two SSDs were writing simultaneously, read speed was around 1.8 GB/s, and the two SSDs wrote at a steady 1.2 GB/s each. That works out at 1.8 GB/s upstream from the Mac, and 2.4 GB/s downstream to it, totalling 4.2 GB/s. That was even better when the drives’ roles were swapped: the fastest SSD then wrote at 2.7 GB/s, as if no other data were being transferred, and the other two SSDs each read at 1 GB/s, giving a total of 4.7 GB/s, which doesn’t seem possible.

Before you try writing those off as being the result of buffering or caching, I should point out that speeds are measured on writing or reading a continuous series of files of random sizes totalling over 50 or 100 GB. They also aren’t entirely unexpected. Looking back at my tests with a display connected, I noted that only disk write speeds were affected, and read performance didn’t change in spite of the video signal.

While Intel is careful to explain how contention between video and data is handled by its Thunderbolt controllers, it paints a much simpler picture when it comes to simultaneous data transfer: “Thunderbolt I/O technology lets you move data between your devices and your computer with 2 channels of 10Gbps flowing both ways (upstream and downstream) at the same time.” That explicitly goes against the notion that separate input and output lines can carry up to 40 Gb/s at the same time in each direction.

Throttling of simultaneous writes is clearly of concern to users, but as we acquire more and faster SSDs, simultaneous reading and writing is more likely to occur in everyday use. You could be streaming video from one SSD to transcode it in your Mac and stream it back to another SSD. On a MacBook Pro, with its limited number of Thunderbolt ports, doing this through a Thunderbolt 4 hub is highly attractive provided its host connection doesn’t prove a bottleneck.

I’m still analysing those test runs, and will report back in the coming week. So far, Thunderbolt 4 hubs are looking like an essential purchase for anyone with more than a couple of Thunderbolt peripherals.