Comment by Tobu

Of course bcachefs can cache data; that was the main draw of bcache as well.

Quoting an example from https://bcachefs.org/GettingStarted/

    bcachefs format /dev/sd[ab]1 \
        --foreground_target /dev/sda1 \
        --promote_target /dev/sda1 \
        --background_target /dev/sdb1
    mount -t bcachefs /dev/sda1:/dev/sdb1 /mnt

This will configure the filesystem so that writes will be buffered to /dev/sda1 before being written back to /dev/sdb1 in the background, and that hot data will be promoted to /dev/sda1 for faster access.

    bcachefs format --help

    --promote_target=(target)
        Device or label to promote data to on read