How does this compare to ZFS? There seems to be some significant overlap in functionality, but ZFS is more mature, so I'm wondering if bcachefs has charm points I'm unaware of.
(Besides the SSD cache persisting across reboots. ZFS has a patch in progress to fix that lack, so it won't last.)
TL;DR: it's based on a stable, pre-existing design (bcache) to avoid design bloat and ensure stability, it's much faster than the competition with similar feature sets (e.g. far lower latency than btrfs), it's very small and lean, and is extensible enough to support most of the common needs (COW, multi-device, replication, compression, encryption, etc etc). Also, bcachefs will hopefully land upstream in Linux and stay there, and unfortunately it seems ZFS will forever have to stay out of tree at this rate (of course, many distros make ZFS very easy to use these days, so if you're comfortable with it, just use that!)
I donated to the project and have been tracking it. I'm not sure how long it will take until Kent sends a request to pull in the code to the upstream kernel, but it's been progressing nicely lately.
While what you wrote was interesting, none of it directly answers any of the questions in the post you followed up to. It would be appreciated to have some answers to what was asked.
(Besides the SSD cache persisting across reboots. ZFS has a patch in progress to fix that lack, so it won't last.)