There were plenty slower drives than the stock. When discussing the PS5's SSD in 'The Road to PS5' Mark Cerny compared it to PS4, stating the stock HDD performance ranged between 50 and 100MB/s. In their GDC Spider-Man post-mortem, Insomniac said that had to go with 20MB/s because some much slower drives were used by gamers to "upgrade" their PS4/Pro.
I didn't mean to say it would be impossible to find slower drives, though I hadn't put much thought into how to best find a drive slower than a 5400rpm 3gbs laptop drive.
On PS5 minimum performance testing is much simpler, you don't have drive geometry and seeks to fixate on so it's really only the sustained read and write performance which you can test in a few minutes.
Off the top of my head, either Sony's test lab or the PS5 would need to catch some things like drives dropping write performance based on whether the SLC cache has been exhausted, which can halve sustained write performance in various places. Then there's empty/full differences in performance, which have been benchmarked ranging from almost none to 20-60%. The same possible range occurs with empty/full average latency, and there are some stark examples of cheap drives with 99th percentile latency spikes in the range of milliseconds for full drives.
Sony's approach, going by Cerny's presentation, seems to be profiling candidate SSDs against the games they have in development. Hopefully that's sufficiently exhaustive, representative enough of what games could be doing years from now, and not biased towards non-fragmented clean/empty test drives.
I think being thorough would take more time per drive if done in a test lab, and possibly beyond the scope of what the console could do at installation time.
I don't think Sony can rescind approval if it turns out drives that seemed fine enough in 2021 start showing problems several years later.
It'll be interesting to see what Sony does but I absolutely not believe they will allow PS5 games to run from NVMe drives that have an overall slower performance profile than the internal solid state drive. I'm sure developers also do not want to be having to deal with that variable performance headache for another generation, it adds work to development and creates another support issue, i.e. users complaining about stutters and so on. Then you get the delightful job of diagnosing their crappy drive. No thanks.
I think Sony would be trying to avoid being so selective that it rules out most of the 7.0 GB/s PCIe 4.0 drives, especially early on when there is only a few of them, while not relaxing standards to the point that games have to account for a worst-case performance that negates a significant portion of the custom SSD's benefits.
This assumes a generally performant custom SSD without its own performance cliffs, although perhaps we should expect some. If they're mostly in-line with other drives, the impact's not as bad as it would be if the expansion and built-in drives tank in different performance regimes--forcing games to assume worse performance throughout. It has more channels than many drives, but it's also a DRAM-less controller of custom design.
How can Sony stop users from putting in dodgy drives? They can publish a list of acceptable models and adopt a 'buyer beware' policy for drives not in it, or is it practical to add such a list to firmware updates?
Sony has to limit operation to SSDs that can run at the stated-to-death “super speed” of the onboard SSD.
If not, games will simply not work as intended when installed on the second SSD and it would be a complete mess.
Either that or we’ll be able to simply store games in that second SSD but unable to play straight from there, and every time we want to play them, they’ll have to move onto the onboard SSD, which at the speeds they run at wouldn’t really take long.
Cerny's presentation seemed to indicate a concern in testing drives to make sure they weren't too alien in performance profile compared to the PS5's own drive. A requirement that games swap onto the main drive would seem like they'd be less worried about that, since that would be a straightforward swap prior to the game running.
Future SSDs with significantly better controllers and/or NAND than Sony's drive could lead to the reverse being preferred. Without good numbers, we can't be sure that the most recent PCIe 4.0 drives aren't already better than a design from a team of undisclosed experience with sometimes finicky SSD implementations.
