I built a franken-server from some bits awhile ago using old disks of different sizes.
The disks were formatted individually with BTRFS (Mostly any FS would do) and then a mergerfs pool setup across all the drives. I got full use of all my old drives. Sizes varied between 1TB and 6TB, gave me 20Tb of usable space.
Mergerfs has no redundancy and no parity although it is designed to work with snapraid.
If a drive is lost only the data on that drive is lost, you just need to replace that one drive, the data on the other drives remains intact. The loss of a drive in a Raid0 or JBOD array, results in the loss of the whole array and all the data.
Some sort of backup is probarbly appropriate.
https://trapexit.github.io/mergerfs/latest/
Might be worth a look.
I needed somewhere to plant files while I reconfigured my ZFS NAS from raidz1 to raidz2 and added an extra drive to compensate for the loss of space. Suprisingly I got away with it!
Mergerfs is quite useful. Now I have a lot of occasional spare storage and somewhere to put snapshots until I can save up for a proper backup drive.
I’ve tried Snapraid but we didn’t get on, I never got the hang of it and lost data as a result.
I think you can use different size disks with ZFS if you use raid10 for the pairs, but the cost would be to high for me. Did I read somewhere that they were working on a way to use different size disks? That would be useful.
Just to add, I’d like BTRFS to be brilliant, so much interesting potential, but I read its deemed safer to use raid10 rather that any variant of raid5. if you’re ever faced with having to rebuild the array, as others have mentioned. Single use is ok as far as I know.