What version is your snapshot from? 2.1.1.26?
I found out more btw: the instructions I was following were for an older btrfs-tools. It should have been Code: mkfs.btrfs --label sailfish --nodesize 4096 --features ^extref,^skinny-metadata new-jolla.img That was the unsupported feature with code (0x100).
mkfs.btrfs --label sailfish --nodesize 4096 --features ^extref,^skinny-metadata new-jolla.img
Seems I learned a lot so far about btrfs.
btrfs scrub start -B /
btrfs balance start -v -dusage=60 -musage=60
btrs-balancer balance
fstrim /
btrfs filesystem defrag -v <file>