2 x 6Tb HDD dedicated to store "iso's", the eye patched ones.
2 x 4Tb ssd for backup.
Ext4 everywhere.
I run this setup for years, maybe even 20 years (with many different disks and sizes over time).
I decided that was time to be more efficient.
I removed the two HDD, saving quite a lot of power, and switched the four sdd to RAID5, Then put BTRFS on top of that. Please note, I am not using btrfs raid feature, but Linux mdadm software raid (which I have been using rock solid for years) with btrfs on top as if on a single drive.
I choose MD not only for my past very positive experience, but specially because I love how easy is to recover and restore from many issues.
I choose not to try zfs because I don't feel comfortable in using out of kernel drivers and I dislike how zfs seems to be RAM hungry.
SSDs for backup? Being rich must be nice. More srsly if you have the upstream pipe for it, remote backups are preferable in case something happens at home.
Right, something like hetzner storage box is a good complement to raid 5 in order to follow the 321 backup rule. You can use rclone to sync your backup to hetzner, and even encrypt it, and they can do automatic snapshots on their end to protect against ransomware.
Looks like a good setup to me. Hdds have a lot of downsides, so if you can afford the extra $20/TB, an all flash array is super useful. Mdadm is rock solid.
The only issue I think is that it's not possible to expand this array like you can on LVM or ZFS, so just watch out for that.
Good point on the expansion. But o am not too bothered about it, as I have always done by moving data around. Takes a while, but leaves you with a set of disks with the old data still there, and it saved my ass a few times in the past. Now I should be fine with good backups, but you never know.
Btrfs on a single storage prevents it from doing auto correction via checksums. I would get rid of the raid5 and do a btrfs raid1 out of these devices. Makes it also easier to swap out devices or expand the raid as btrfs supports arbitrary sizes.
One of the main features of file systems like btrfs or ZFS is to store a checksum of each file and allow you to compare that to the current file to notice files becoming corrupted. With a single drive all btrfs can do then is to inform you that the checksum doesn't match to the file any longer and that it is thus likely corrupted, but on a btrfs raid it can look at one of the still correct duplicates and heal the file from that.
IMHO the little extra space from mdadm RAID5 is not worth the much reduced flexibility in future drive composition compared to a native btrfs raid1.