Linux

You can do it but I wouldn't recommend it for your use-case.

Caching is nice but only if the data that you need is actually cached. In the real world, this is unfortunately not always the case:

1. Data that you haven't used it for a while may be evicted. If you need something infrequently, it'll be extremely slow.
2. The cache layer doesn't know what is actually important to be cached and cannot make smart decisions; all it sees is IO operations on blocks. Therefore, not all data that is important to cache is actually cached. Block-level caching solutions may only store some data in the cache where they (with their extremely limited view) think it's most beneficial. Bcache for instance skips the cache entirely if writing the data to the cache would be slower than the assumed speed of the backing storage and only caches IO operations below a certain size.

Having data that must be fast always stored on fast storage is the best.

Manually separating data that needs to be fast from data that doesn't is almost always better than relying on dumb caching that cannot know what data is the most beneficial to put or keep in the cache.

This brings us to the question: What are those 900GiB you store on your 1TiB drive?

That would be quite a lot if you only used the machine for regular desktop purposes, so clearly you're storing something else too.

You should look at that data and see what of it actually needs fast access speeds. If you store multimedia files (video, music, pictures etc.), those would be good candidates to instead store on a slower, more cost efficient storage medium.

You mentioned games which can be quite large these days. If you keep currently unplayed games around because you might play them again at some point in the future and don't want to sit through a large download when that point comes, you could also simply create a new games library on the secondary drive and move currently not played but "cached" games into that library. If you need it accessible it's right there immediately (albeit with slower loading times) and you can simply move the game back should you actively play it again.

You could even employ a hybrid approach where you carve out a small portion of your (then much emptier) fast storage to use for caching the slow storage. Just a few dozen GiB of SSD cache can make a huge difference in general HDD usability (e.g. browsing it) and 100-200G could accelerate a good bit of actual data too.

Doing this with ZFS is called L2ARC and is very easy to set up.

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.

For many games, the loading times are not thaaaat different when comparing HDD vs SSD vs NVME. (Depends on how impatient you are tbh.) And it barely affects FPS.

The biggest appeal of NVME/SSD for me is having a snappy OS.

So I would put your rarely played games on a cheap, big HDD and keep your OS and a couple of the most frequent games on the NVME. (In the Steam interface you can easily move the games to a new drive)

I find it to be a much simpler solution than setting up a multi tiered storage system.

Some sources:

https://www.legitreviews.com/game-load-time-benchmarking-shootout-six-ssds-one-hdd_204468

https://www.phoronix.com/review/linux-gaming-disk/3

https://www.pcgamer.com/anthem-load-times-tested-hdd-vs-ssd-vs-nvme/

Have you also tried using Bleachbit to free up storage space?

...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.

How much storage do you need? If it's just 2TB - or if you're just future proofing it and don't anticipate needing more than 2TB within the next few years - I'd pick one SSD. Even the cheaper ones will give you a better performance than an SSD + HDD combo.

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