On Sun, Oct 17, 2021 at 6:37 AM Jim Davis [off-list ref] wrote:

I've been trying some experiments with raid1c3 on a qemu virtual
machine running Ubuntu 21.10.  Choosing btrfs initially as the root
file system during installation works just fine.

Adding a new virtual disk to btrfs root file system and running btrfs
balance with -mconvert=raid1 and -dconvert=raid1 works too -- the vm
reboots with no problems.

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

message.  Any ideas?

Support for the raid1cX modes was added to GRUB in GRUB 2.06. The
Debian family (including Ubuntu) has not upgraded yet, nor have they
backported the support to their custom release of GRUB 2.04 yet. I
would suggest filing a bug with Ubuntu to backport the following
commit: https://git.savannah.gnu.org/cgit/grub.git/commit/grub-core/fs/btrfs.c?id=495781f5ed1b48bf27f16c53940d6700c181c74c


-- 
真実はいつも一つ！/ Always, there's only one truth!