Mac SSD Performance
Apple’s exhorbitantly-priced Mac storage comes with impressive benchmark claims but has some serious, hidden bottlenecks in certain scenarios, as do other SSDs. David Harry illustrated this clearly in a video, using Activity Monitor as a novel window into dramatic SSD slowdowns when moving large amounts of data to an Apple internal SSD from a third-party Thunderbolt 5 SSD.
He actually finds that the external SSD, connected via Thunderbolt 4, is faster at reading than the MacBook Pro’s internal one. Write performance drops once the SLC cache is full.
In normal use, there are three potential causes of reduced write speed in an otherwise healthy SSD:
- thermal throttling,
- SLC write cache depletion,
- the need for Trimming and/or housekeeping.
[…]
To achieve their high storage density, almost all consumer-grade SSDs store multiple bits in each of their memory cells, and most recent products store three in Triple-Level Cell or TLC. Writing all three bits to a single cell takes longer than it would to write them to separate cells, so most TLC SSDs compensate by using caches. Almost all feature a smaller static cache of up to 16 GB, used when writing small amounts of data, and a more substantial dynamic cache borrowed from main storage cells by writing single bits to them as if they were SLC (single-level cell) rather than TLC.
[…]
Keep ample free space on the SSD so the whole of its SLC write cache can be used.
The price on added memory and SSD capacity on the otherwise excellent Mac Mini is horrible. I wanted to avoid paying the apple tax on that storage, so I am using an external SSD and it means I can get twice the storage for less than half the price. But I wanted to find out what it would mean for performance, and I couldn’t believe what I found.
The Mac mini also has Thunderbolt 4, but he finds it limiting performance. The internal 256 SSD seems to be slow, despite what was reported earlier about it using multiple chips like the larger versions.
Previously:
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This is the difference between an entry level product like the Mac mini (cost optimized) vs. a Pro version like a Mac Studio Ultra (memory bandwidth optimized).
All manufacturers, including Apple, should show full specifications of what they sell. Full means full. Complete. All (absolutely all) included.
The other thing is that even if the pricing was reasonable (and it isn’t; there is zero way to justify the price), at this point, the SSDs (regardless of number of chips) Apple includes with all of its machines are not as fast as the NVMe’s you can buy from Crucial or WD or Samsung — like they just aren’t as good. I talk to so many people who still think that it is 2015 and that the only people selling or shipping fast NVMe drives in new machines are Apple, when Apple has frankly become mid-tier, irrespective of how much you spend on that 8TB upgrade.
The problem, of course, is that there are consequences to not using the internal disk, so no matter how good it is, you have to anticipate uses that would otherwise have been fine for an external SSD. I serve stuff off my Mini's internal disk to my LAN, just because I can, but that's because TB4 is the bottleneck and even at 10 Gbps I still max out the throughput of the internal drive (with larger files, because there's a latency penalty for protocol overhead). TB5 changes this equation and once drives start appearing it'll be well worth considering booting from them as a way to make full use of that bandwidth.
The fileprovider framework (or what it is called to sync Google drive and similar) on Macos insists on using internal drive only.
