...depends what your use pattern is, but I doubt you'd enjoy it.

The problem is the cached data will be fast, but the uncached will, well, be on a hard drive.

If you have enough cached space to keep your OS and your used data on it, it's great, but if you have enough disk space to keep your OS and used data on it, why are you doing this in the first place?

If you don't have enough cache drive to keep your commonly used data on it, then it's going to absolutely perform worse than just buying another SSD.

So I guess if this is 'I keep my whole steam library installed, but only play 3 games at a time' kinda usecase, it'll probably work fine.

For everything else, eh, I probably wouldn't.

Edit: a good usecase for this is more the 'I have 800TB of data, but 99% of it is historical and the daily working set of it is just a couple hundred gigs' on a NAS type thing.

I used to run an HDD with an SSD cache. It's deffo not as fast as a normal SSD. NVMe storage is also very cheap. You can get a 2tb NVMe for the same price as SATA.

In all honesty, I'd just keep things simple and go for a SSD.

Apple tried it a decade ago. It was called the Fusion Drive. It performed about as well as you’d expect. macOS saw the combined storage, but the hardware and OS managed the pair as a single unit.

If there’s a good tiered storage daemon on your OS of choice, go for it!

Currently have 2 1tb NVME's over around 6 tb of HDDs, works really nice to keep a personal steam cache on the HDD's in case I pick up an old game with friends, or want to play a large game but only use part of it (ie cod zombies).

Also is super helpful for shared filesystem's (syncthing or NFS), as its able to support peripheral computers a lot more dynamically then I'd ever care to personally configure. (If thats unclear, I use it for a jellyfin server, crafty instance, some coding projects - things that see heavy use in bursts, but tend to have an attention lifespan).

Using bcachefs with backups myself, and after a couple months my biggest worry is the kernel drama more than the fs itself

Back when SSDs were expensive and tiny they used to sell hybrid drives which were a normal sized HDD with a few gigs of SSD cache built in. Very similar to your proposal. When I upgraded from a HDD to a hybrid it was like getting a new computer, almost as good as a real SSD would have been.

I say go for it.

If it's all Steam games then you could just move games around as needed, no need for a fancy automatic solution.

What I personally have is an NVME SSD for the games that need the maximum performance and/or I play a lot, and a slower SATA SSD for the other games where it doesn't really matter.

Also, depending on the games you are playing, many expect to be able to stream assets and other data at SSD speeds, so your experience might be really bad.

I used to do this all the time! So in terms of speed bcache is the fastest, but it’s not as well supported as lvm cache. IMHO lvm cache is plenty fast enough for most uses.

Is it going to be as fast as a NVME ssd? Nope. But it should be about as fast as a SATA ssd if not a little slower depending on how it’s getting the data. If you’re willing to take that trade off it’s worth it. Though anything already cached is going to be accessed at NVME speeds.

So it’s totally worth it if you need bigger storage but can’t afford the SSD. I would go bigger in your HDD though, if you can. Because unless you’re accessing more than the capacity of your SSD frequently; the caching will work extremely well for both reads and writes. So your steam games will feel like they’re on a SSD, most of the time, and everything else you do will “feel” snappy too.