Thunderbolt on the M1 Mac mini – When 2 Actually Does Equal 4

By OWC Tim

I have read a lot of complaints about the new M1 based Macs only having two Thunderbolt ports. Having only two ports doesn’t seem like enough for professional use. Both my 2019 16 inch MacBook Pro and my 2018 Mac mini have four Thunderbolt ports, and I can’t imagine using a Mac with only two.

My desire for more than two ports comes from using bus-powered devices like the OWC Envoy Pro EX and the OWC Thunderbolt 3 10G Ethernet Adapter. Both of them have only one Thunderbolt connection, so they must either be connected to a dedicated Thunderbolt port on my Mac or be at the end of a chain of devices.

On Intel Macs, each pair of Thunderbolt ports is actually one bus

My other concern is that both my Macs only really have two Thunderbolt buses. Each pair of Thunderbolt ports on my Intel Mac is connected to a single bus powered by a dedicated controller chip. Since the two ports in a pair are connected to one bus, they share the Thunderbolt bandwidth. The total throughput of both ports together on a single bus is limited to 2,800 MB/sec.

Infographic showing Thunderbolt controllers on a MacBook Pro
On a MacBook Pro, two Thunderbolt ports share one bus (Thunderbolt controller)

You can confirm the number of Thunderbolt buses in your Mac by viewing the System Report window (select About This Mac in the Apple menu and then click the System Report… button). Then click on Thunderbolt in the column on the left. Here is what I see on my 2019 16 inch MacBook Pro, showing the two Thunderbolt buses. (4 ports = 2 pairs of ports = 2 Thunderbolt buses)

2019 16-inch MacBook Pro System Report
2019 16-inch MacBook Pro System Report

Things are different on the M1 Macs

We received our first M1 Mac last week, and I have been using it ever since to debug and test the ARM version of the SoftRAID driver. The first thing I noticed was that the System Report indicated that there were two Thunderbolt buses, just like my 2019 16 inch Mac Book Pro.

2020 M1 mac mini System Report
2020 M1 mac mini System Report

Wow, that’s unlike any other Thunderbolt-equipped Mac I’ve ever used. System Report indicates that there are two Thunderbolt buses, one for each of the Thunderbolt ports!! This means that each port has its own dedicated bus and doesn’t have to share its bandwidth with any other port. Each one will have 2,800 MB/sec all to itself. I was a bit skeptical and wanted to test this to be certain.

Infographic showing Thunderbolt controllers on an M1 Mac mini
On an M1 mac mini the two Thunderbolt ports each have their own bus (Thunderbolt controller)

Testing the bandwidth on the two Thunderbolt ports of the M1 Macs

I used the AJA System Test Lite application, a file I/O benchmarking application, to test whether these two ports were indeed connected on separate Thunderbolt buses. I set up two OWC ThunderBlades and connected each to a separate Thunderbolt port on the M1 Mac mini. Each ThunderBlade contains 4 NVMe SSDs, so I created a stripe (RAID 0) volume using all eight blades to test performance. If the two Thunderbolt ports share bandwidth, I will not be able to read or write faster than 2,800 MB/sec, the limit of a single Thunderbolt bus. If the two ports are indeed on separate Thunderbolt buses, I will see faster performance.

M1 Mac mini AJA Performance Test Results
M1 Mac mini AJA Performance Test Results

The results using AJA System Test show that the 2 Thunderbolt ports of the M1 Mac are indeed on separate Thunderbolt buses as I get over 3,500 MB/sec reading from my stripe volume.

What about only being able to connect two Thunderbolt devices to the M1 Macs?

I may have the same bandwidth from two Thunderbolt ports on the M1 Macs as I have on the Intel Macs, but I still can only connect two devices directly to this new Mac. Isn’t this going to be a problem? It turns out that Thunderbolt 4 saves us here.

The move from Thunderbolt 3 to Thunderbolt 4

The M1 Macs are the first Macs with support for the Thunderbolt hub feature found in Thunderbolt 4. Thunderbolt 4 requires computer makers to implement a whole set of features, most of which were optional in Thunderbolt 3. These features have actually been found in almost all Macs that Apple has shipped in the past four years, so the move to Thunderbolt 4 isn’t a huge change for Mac users.

These features include 40 Gb/sec Thunderbolt ports, support for charging laptops over Thunderbolt, and protection from malicious hardware that might try and snoop computer memory over Thunderbolt. The one feature in Thunderbolt 4 new to the Mac is the support of Thunderbolt Hubs.

What is a Thunderbolt Hub?

A Thunderbolt hub, for instance, the OWC Thunderbolt Hub, gives you more ports to connect Thunderbolt devices. In the same way that Intel Macs have two Thunderbolt ports connected to a single Thunderbolt bus, a Thunderbolt Hub allows you to have three Thunderbolt ports connected to a single Thunderbolt port on a Mac. 

In both cases, you can connect a chain of devices to each of the downstream Thunderbolt ports (either the two ports on your Intel Mac or the three ports on a Thunderbolt Hub). This allows you to connect more devices that only have a single Thunderbolt connection and improves stability by separating storage devices from other less critical devices. (I usually suggest that displays being connected over Thunderbolt not be combined on the same chain as storage devices to improve the reliability of your mounted volumes.)

So with an M1 Mac and a single Thunderbolt Hub, I can have the stability and usability which comes from four Thunderbolt ports currently found in my Intel Macs and also have the high bandwidth which comes with two separate Thunderbolt buses. Combine that with the lack of fan noise and better battery life, and the move to an M1 Mac now looks very enticing.

Now, if only I can persuade my boss to get me a new M1 MacBook Pro to use as my development machine…