Thunderbolt May Not Be As Fast As You Think

Thunderbolt is an amazingly fast protocol for connecting computers to storage devices. However, even though it has potentially blazing speed, if you don’t understand how it works, you may not be getting the speed you expect.

A MATTER OF PLUMBING

An effective way to think about moving data between storage and a computer is to think about plumbing. Yup. Pipes and water pressure.

Thunderbolt defines the size of the pipe, but it doesn’t determine water pressure. The type of storage hardware and how it is configured determines the water pressure.

NOTE: Um, no. Data is not water, this is an analogy. Your computer is not getting wet.

THUNDERBOLT – The Size of the Pipe

Thunderbolt is the name of a hardware interface, co-developed by Intel and Apple, that connects external devices to a computer. It was first released in 2011. It combines two signals into one cable (or “pipe”): PCIe, for data, and DisplayPort, for monitors. It also has the ability to carry power; though the wattage varies.

Thunderbolt is optimized for rapid transfer of large files, which is why it is so useful for video editing. There are four versions of Thunderbolt, with a fifth version rumored but not announced:

The principle difference between versions 1, 2 and 3 is the size of the pipe they create. (The bandwidth of Thunderbolt 3 and 4 is identical.) However, that pipe is split into two parts: one for data and the other for monitor video.

Illustration of Available Data Rates

Protocol Total Speed Reserved for Video Maximum for Data
Thunderbolt 1 10 Gbps
(1.25 GB/s)
Min. 6 Gbps ~4 Gbps
(~500 MB/s)
Thunderbolt 2 20 Gbps
(2.5 GB/s)
Min. 8 Gbps ~12 Gbps
(~1250 MB/s)
Thunderbolt 3 or 4 40 Gbps
(5.0 GB/s)
Min. 8 Gbps ~32 Gbps
(~2600 MB/s)

NOTES:

The amount of bandwidth required by video is dependent upon the monitor size, bit depth, refresh rate, and the number of monitors connected. It is entirely possible to fill almost all of a Thunderbolt pipe feeding monitors, leaving almost no bandwidth for data.

Monitor Bandwidth Requirements

Monitor size Pixel size Approx. Bandwidth
for 1 Monitor
(2X for two)
2k 2560 x 1600 ~8 Gbps
4K 3840 x 2160 14-15 Gbps
5K 5120 x 2880 22 Gbps
6K 6016 x 3384 31 Gbps

NOTES:

So, as an example, if you are running a 4K monitor at 60 Hz over Thunderbolt 3, the maximum available data bandwidth would be 40 – 15 = 25 Gbps; or roughly 3 GB/s. After decoding this data rate drops to about 2.6 GB/s. Running two 4K monitors over the same Thunderbolt 3 or 4 cable would reduce the maximum data bandwidth to roughly 10 Gbps, or roughly 1 GB/second.

Photo courtesy of Azamat Esenaliev, Pexels.com.

BUT WAIT, THERE’S MORE!

To quickly summarize: a large chunk of Thunderbolt bandwidth is devoted to video and is inaccessible for data transfer.

However, Thunderbolt only defines the size of the pipe. Your actual storage devices determine the maximum amount of data (the “water pressure”) that can flow through the pipe each second. Here’s a table that illustrates some of these differences.

Rough Data Transfer Rates for Typical Storage

Device Configuration Data Rate
Spinning hard disk (HDD) N/A 125 – 225 MB/s
2 HDD RAID RAID 0 ~350 MB/s
2 HDD RAID RAID 1 ~175 MB/s
4 HDD RAID RAID 5 ~575 MB/s
4 HDD RAID RAID 6 ~350 MB/s
8 HDD RAID RAID 5 ~1,235 MB/s
8 HDD RAID RAID 6 ~1,050 MB/s
PCIe SSD N/A ~400 MB/s
2 PCIe SSD RAID RAID 0 ~800 MB/s
2 PCIe SSD RAID RAID 1 ~400 MB/s
4 PCIe SSD RAID RAID 4 1,200 MB/s
NVMe SSD N/A ~2,500 MB/s
2 NVMe SSD RAID RAID 0 ~2,650 MB/s (limited by Thunderbolt bandwidth)
2 NVMe SSD RAID RAID 1 ~2,500 MB/s

NOTES:

Another weirdness of storage is that transfering large files is much faster than transferring smaller files. For example, transferring a 10 GB file will come close to matching these maximum speeds. Transferring, say, fifty 1 MB files to an HDD will average about 15 MB/sec.

SUMMARY

Now that our brains have exploded, here are the key takeaways:

Thunderbolt is an amazing protocol and essential to many video workflows. However, it isn’t a magic box. Once you understand what affects the speeds it can transfer data, you can better plan for how to use it.

EXTRA CREDIT

In September, 2021, Intel leaked a new version of Thunderbolt, dubbed “Thunderbolt 5,” that promises an 80 Gbps bandwidth! It is expected to use some form of USB-C for connectivity. However, since then, no other information was released and no current products support it.


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14 Responses to Thunderbolt May Not Be As Fast As You Think

  1. Philip Cutting says:

    I never knew that! Thanks Larry!

    • Larry says:

      Philip:

      Thanks! I remain a huge fan of Thunderbolt – it is optimized for fast transfer of very large files and works great for editing. But there is a lot more to it when you look below the marketing hype of “FAST!”.

      Also, while USB-C is fast, it’s optimized for transferring smaller files. This is not to say USB-C is bad. It isn’t. But it isn’t the same as Thunderbolt.

      Larry

  2. Lee says:

    Good summary Larry!

    One other effect with Thunderbolt is the channels. Just because there are multiple Thunderbolt (USB-C) ports doesno’t mean each runs separately. One thing to be aware of is ovverloading the Thuderbolt ports. The computer (oh, say a Mac Studio with M! Max has 4 Thuderbolt ports on the back. If you check the System report, there are 4 Thunderbolt independent buses. Each operating separate PCIe bus interfaces. BUT, if you attach a device, say an OWC Gemini (2 disk Thunderbolt 3 RAID) ti has 2 ports for daisychaining. But ony 1 bus so daisychaining adds load but not independently running busses so all bandwidth is subtracted (the water pressure has too many holes in the pipe).

    It’s another factor in optimizing your usage of Thunderbolt.

    • Larry says:

      Lee:

      Good point. If you daisy-chain multiple devices, say Thunderbolt 3, they SHARE the 40 Gbps pipe between all connected devices.

      If you have a Mac Studio and plug two devices each into their own port on the back of the Mac Studio, each device has the full bandwidth of Thunderbolt 4 available to it.

      Larry

  3. Al says:

    Hi Larry – I bet you have done this before, but in terms of best practices; I would love to see an article on optimum placement of project file (main internal drive vs external), file output/render, and video assets.
    If you have the ability to segregate ea component, what is ideal?

    • Larry says:

      Al:

      It depends… Optimium is defined by the size of your project. Most of the time, media files are too big to fit on the internal drive, so I recommend they be stored externally.

      Cache files can be stored on the internal drive – unless you are doing a super-long film, in which case you won’t have room.

      If your project is small and media limited, you can store everything to the internal drive for editing, then move it to external storage for archiving. If not, keep everything on an external drive.

      Larry

  4. George Henderson says:

    Thanks Larry, this was incredibly informative!
    So much I didn’t know I needed to know about Thunderbolt.

    • Larry Jordan says:

      George:

      I’m glad you liked it.

      Yeah, I always learn something when I write these tutorials. My initial thought is: “Let’s write something simple about ‘x’.” Only to discover that “x” isn’t simple at all.

      We all learn.

      Larry

  5. Clayton Moore says:

    Very good and very useful!

  6. Russ Anderson says:

    Larry, Great article, but could you check the Illustration of Available Data Rates for the Thunderbolt 2 and 3/4.

    12 Gbps = 1500 MB/s
    32 Gbps = 4000 MB/s

    Thanks, Russ

    • Larry says:

      Russ:

      The problem is that all Thunderbolt data is encoded for transfer, then decoded at the end for storage. That encoding/decoding has an overhead. While your numbers are mathematically accurate, that encode/decode limits the actual throughput speed of Thunderbolt. However, I was unable to get a definitive understanding of exactly how much that overhead is.

      Larry

  7. Andrew Howe says:

    I have been researching getting a new external high capacity nvme drive. You can do this via a big single or multiple smaller drives in RAID and the economics on this is always shifting.

    I was thinking about this conclusion you came to: “NVMe SSDs, when combined as a RAID, exceed the data bandwidth provided by Thunderbolt. There’s no advantage, currently, to using multiple NVMe devices in a Thunderbolt RAID.”

    This is true when it comes to max throughput. Even single drives can exceed the Thunderbolt bandwidth. However, in some of the tests I have watched or read, the maximum throughput has not been maintained by the SSDs. This can be due to the chip type, but is more commonly is a thermal throttling issue. I was wondering if RAID would give a different result in terms of performance stability. I suppose, depending on the enclosure, it could be better or worse. I haven’t seen this kind of test done – have you come across one?

    • Larry says:

      Andrew:

      My current understanding is that an NVMe RAID won’t exceed the performance of a single NVMe when connected via Thunderbolt. (Apple uses a faster connection to create it’s NVMe RAID, which is the internal drive of most M1/M2 Macs.

      However, a PCIe SSD RAID makes a lot os sense. You can increase total storage into multiple terabytes, have a transfer speed that matches a Thunderbolt 4 pipe, and have replaceable SSDs in case one dies. I’m hoping to buy one of these in the next few weeks for testing. They aren’t cheap, but they are far more flexible than a single NVMe drive.

      Just my thoughts.

      Larry

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