Thunderbolt 5 SSDs are promising, but fill their cache quicker
As the first batches of Thunderbolt 5 SSDs are starting to ship, this is a good time to take stock of what we have seen so far. Here I include some current results obtained from testing one of these new products.
So far, products that have already been released or are well into pre-order include:
OWC Envoy Ultra,
LaCie Rugged SSD Pro 5,
Sabrent Rocket XTRM5.
Starting prices for these are under $/€/£ 400 for 2 TB, making them a little more expensive than better USB4 or Thunderbolt 3 equivalents. However, there’s no sign of any empty enclosures to compare with those available for USB4, so self-assembly isn’t yet an option, and it’s not clear whether or when it might become one.
Mac support
The only Macs that currently support TB5 are the latest MacBook Pro and Mac minis equipped with M4 Pro or Max chips. Although they’re claimed to support full-speed TB5 performance when running macOS Sequoia 15.0, problems have been reported in achieving that, at least with TB5 docks and hubs. If you intend using any TB5 peripheral, then you’d best start with 15.3, which has been reported as solving at least some of those problems. This may also explain some of the anomalies in SSD performance that have been claimed by a few early testers.
Other Apple silicon Macs should run TB5 SSDs in USB4 40 Gb/s mode, which should still be significantly faster than TB3. Intel Macs don’t support TB5 or USB4, though, so they’re most likely to fall back to run them as USB 3.2 Gen 2, at 1 GB/s, which would be a deep disappointment for the cost.
Benchmarks
Beware of claimed performance of TB5 SSDs by their manufacturers and in product reviews. Testing them isn’t as straightforward as with slower products.
For a start, quoted results are often taken from apps such as Blackmagic Disk Speed Test and AmorphousDiskMark. Although both have their uses, they also have their limits, notably that they only measure read and write speed for one test size, normally 5 GB in the former and 1 GiB in the latter. As the graph below shows, there are substantial differences in speed between different sizes.
The upper pair of unbroken lines show read and write speeds when operating in TB5 mode over 80 Gb/s to the Mac mini M4 Pro, and the pair below them with broken lines shows speeds when operating in USB4 mode over 40 Gb/s to a MacBook Pro M3 Pro. Calculated overall read/write speeds were 5.2/5.5 GB/s for TB5, and 3.1/3.1 GB/s for USB4. For comparison, Blackmagic returned 4.8/5.2 GB/s, and Amorphous 6.8/5.3 GB/s. Needless to say, the latter is the result being quoted by the manufacturer, despite its write speed looking highly suspect.
Highest read and write speeds were measured with 400 MB size, and there were only small differences once sizes exceeded 600 MB. However, speeds for files below 10 MB were considerably less than 100 MB and larger. Fortunately, Blackmagic Disk Speed Test and AmorphousDiskMark both use sizes in the more linear range above 600 MB, but their results don’t take into account smaller file sizes, which are more common in many real-world circumstances. As the 80th centile write speed was 5.62 GB/s, the write speed reported by Amorphous of 6.8 GB/s doesn’t appear representative.
Caching effects
All those benchmark results are subject to a major caution: they don’t provide any information on caching, used by most faster SSDs to improve performance. This typically uses part of the memory in SLC mode, sacrificing capacity for speed. I’ve seen a figure of 50 GB of cache quoted for one TB5 SSD that I’ve tested, and sure enough, once 50 GB has been written to it, its write speed drops from around 5.5 to 1.4 GB/s.
Few are likely to write more than 50 GB to an SSD in a single continuous session, but for those that do, it’s important to know when the SLC cache is likely to become full, and for write speed to fall to little better than USB 3.2 Gen 2. Neither Blackmagic Disk Speed Test nor AmorphousDiskMark can measure that for you.
Overall impressions
Thunderbolt 5 SSDs are starting to realise their promise of significantly faster read and write performance than even USB4 SSDs. Although TB5 SSDs are supported by a small range of the latest Macs, they should still be faster than TB3 when they fall back to USB4 on other Apple silicon Macs. However, if they’re to be used with Intel Macs, the likelihood is that they will fall further back to USB 3.2 Gen 2.
If you’re likely to stream very large quantities of data to them, more than about 50 GB, then you’ll need to obtain an estimate of the size of their SLC cache, and of their write performance when that is full, or you could be in for a big disappointment. The trouble with TB5 SSDs is that they’re sufficiently fast to fill their cache very quickly, in this case in around 10 seconds when writing at full speed.
