On Fri, 15 Oct 2021 at 19:09, Jim Davis [off-list ref] wrote:

Adding another new virtual disk to btrfs root file system and running
btrfs balance again, with -mconvert=raid1c3 and -dconvert=raid1c3 and
then rebooting doesn't work: the vm drops into grub rescue with a
cryptic

error: unsupported RAID flags 202

Grub needs new code for new raid levels in btrfs. Btrfs raid1c3/4
support was introduced in 2019:
https://git.savannah.gnu.org/cgit/grub.git/commit/?id=495781f5ed1b48bf27f16c53940d6700c181c74c

which is released as grub 2.06, but Ubuntu 21.10 ships grub 2.04.


Lukas