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
|Min. 6 Gbps||~4 Gbps
|Thunderbolt 2||20 Gbps
|Min. 8 Gbps||~12 Gbps
|Thunderbolt 3 or 4||40 Gbps
|Min. 8 Gbps||~32 Gbps
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|
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.
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
|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|
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.
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.
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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