I’ve been using single-disk btrfs for my rootfs on every system for almost a decade. Great for snapshots while still being an in-tree driver. I also like being able to use subvolumes to treat / and /home (maybe others) similar to separate filesystems without actually being different partitions.
I had used it for my NAS array too, with btrfs raid1 (on top of luks), but migrated that over to ZFS a couple years ago because I wanted to get more usable storage space for the same money. btrfs raid5 is widely reported to be flawed and seemed to be in purgatory of never being fixed, so I moved to raidz1 instead.
One thing I miss is heterogenous arrays: with btrfs I can gradually upgrade my storage one disk at a time (without rewriting the filesystem) and it uses all of my space. For example, two 12TB drives, two 8TB drives, and one 4TB drive adds up to 44TB and raid1 cuts that in half to 22TB effective space. ZFS doesn’t do that. Before I could migrate to ZFS I had to commit to buying a bunch of new drives (5x12TB not counting the backup array) so that every drive is the same size and I felt confident it would be enough space to last me a long time since growing it after the fact is a burden.
For what it’s worth, the “Download & transfer via USB” feature was applying DRM locked to the key of the specific Kindle device you select, giving you a file that’s incompatible with other devices even if they’re kindles linked to the same Amazon account. For many publishers it also gives files with drastically lower image quality than the Kindle app: about one-fourth to one-third the file size. For a couple examples, a 368MB KFX manga volume has a 125MB AZW3 file and an 8.0MB KFX light novel has a 2.2MB AZW3 file. Those smaller AZW3 files are also similar in size to DRMed EPUB files of the same books from other markets like Kobo and Google Play, so I expect it’s a deliberate choice to limit the quality of formats that are more trivial to strip DRM from.
The best way I’ve found to make personal backups of owned Kindle content is to use a rooted Android device to download everything through the Kindle app, copy the KFX files to a computer, extract the key in a root shell, and then use DeDRM tools on those files with that key.
A quick and dirty shell command I’ve used for that purpose is
egrep -ao 'dsn[0-9a-f]{32}' /data/data/com.amazon.kindle/databases/map_data_storage.db. The key is 32 hex characters.Having a rooted Android device in the first place is the biggest hurdle for being able to do that. This new jailbreak should make it possible to do something similar with e-ink kindles instead.