On Mon, Oct 18, 2021 at 12:14:55AM +0200, waxhead wrote:

Neal Gompa wrote:

quoted

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

quoted

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

This would be useful information to have on the STATUS page for 
RAID1c3/RAID1c4 as well. I am on Debian and had to discover this the 
hard way when I believed that Grub 2.04 *had* this backported , which it 
of course has not...

This information is too distro specific, the status on btrfs wiki would
get stale and unreliable, I doubt anybody from ubuntu community would go
there to fix it. There used to be distro specific pages on the k.org
wiki but all got deleted because of no interest to keep them up to date.

The only time-proof advice is "make sure your distro and system supports
all features you enable".