[Bug 1660882] [NEW] Can't boot from BTRFS RAID1

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Ubuntu 16.10

Steps to reproduce:

1. Install Ubuntu with a BTRFS root partition (/dev/sdb2 on my system).

2. After installation, convert to RAID 1 by adding another partition to the filesystem:
    $ btrfs device add /dev/sdb3 /
    $ btrfs filesystem balance /

3. Reboot.

4. Stuck at initramfs prompt.

To get the system to boot I have to edit /boot/grub/grub.cfg and replace all instances of this:
    rootflags=subvol=@
with this:
    rootflags=subvol=@,device=/dev/sdb2,device=/dev/sdb3

The workaround can be applied permanently by editing /etc/grub.d/10_linux and replace this:
    rootflags=subvol=${rootsubvol}
with this:
    rootflags=subvol=${rootsubvol},device=/dev/sdb2,device=/dev/sdb3

Then run update-grub

** Affects: grub (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: bot-comment
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Can't boot from BTRFS RAID1
https://bugs.launchpad.net/bugs/1660882
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