Change default btrfs subvolume - GRUB problem

Question

grub-mkconfig  creates a grub.cfg with this kernel command line:
linux /vmlinuz-linux root=UUID=88e4e9db-0d5d-4055-bb36-6c4bc3454cb3 rw rootflags=subvol=SB0 quiet video=hyperv_fb:1920x1080
my problem lies in rootflags=subvol=SB0 where is named a specific btrfs subvolume.
I would like no subvolume be specified in grub.cfg so that the default one will be mounted as root filesystem.
The reason is I have arch installed into btrfs subvols and I would like to choose which arch variant to boot simply changing default subvol.
(I've used the installation tips found at http://www.funtoo.org/BTRFS_Fun )
This is btrfs root :
~/bin > mount /dev/sda2 -o subvolid=0 /mnt
~/bin > btrfs filesys show /mnt
Label: none uuid: 88e4e9db-0d5d-4055-bb36-6c4bc3454cb3
Total devices 1 FS bytes used 3.47GiB
devid 1 size 126.81GiB used 6.04GiB path /dev/sda2
btrfs-progs v3.19.1
/etc/fstab :
~/bin > cat /etc/fstab
# /etc/fstab: static file system information
# <file system> <dir> <type> <options> <dump> <pass>
# /dev/sda2
UUID=88e4e9db-0d5d-4055-bb36-6c4bc3454cb3 / btrfs rw,relatime,space_cache 0 0
# /dev/sda1
UUID=df2d9bbc-ab0a-42a5-96ae-7c90f0c15a0e /boot ext3 rw,relatime,data=ordered 0 2
These are the subvolumes:
~/bin > btrfs subvol list /mnt
ID 260 gen 5129 top level 5 path SB0
ID 264 gen 4975 top level 260 path SB0/var/lib/machines
ID 265 gen 5092 top level 5 path SB1
SB1 is a snapshot of SB0 and both contain an arch installation
SB1 is default one:
~/bin > btrfs subvol get-default /mnt
ID 265 gen 5092 top level 5 path SB1
but if I run grub-mkconfig it picks up SB0, in my opinion the wrong one.
Please note that SB0 is currently mounted as system root:
~/bin > findmnt -o SOURCE,TARGET,FSTYPE,FSROOT /
SOURCE TARGET FSTYPE FSROOT
/dev/sda2[/SB0] / btrfs /SB0
Is it possible to tell to grub-mkconfig to not name any specific subvol ?
If you look into: /etc/grub.d/10_linux
and search for "btrfs"  it may give an hint, but I don't understand it.
many thanks in advance
Andrea
EDIT: of course, if I change kernel command line by hand (:s/SB0/SB1) it works as expected, but it's not I'm looking for, being grub.cfg an automatically generated file.
Last edited by Aklo (2015-04-28 11:11:56)

Answer 1

I don't use GRUB anymore, but I don't recall it requiring a specified subvolume to boot, however if you've installed to a specific subvolume (SB0 in your case), then you will need to to tell GRUB where to look more specifically than just the device and partition (/dev/sda2).
What might accomplish what you're asking for is to set up seaparate boot entries for the respective subvolumes / installation variants.  Basically manually add another "menuentry" in /boot/grub/grub.cfg and change the subvolume specified:
/boot/grub/grub.cfg
menuentry "Arch Linux - SB0" {
linux /boot/vmlinuz-linux root=/dev/sda2 rootflags=subvol=SB0
menuentry "Arch Linux - SB1" {
linux /boot/vmlinuz-linux root=/dev/sda2 rootflags=subvol=SB1
You'd need to fiddle with /etc/fstab a then since I'm pretty sure something would freak out if you tried to mount two subvolumes to / at the same time (e.g., duplicate entries with different subvolumes specified).  A much simpler solution would be separate root partitions with shared /boot and /home partitions.  Like so (subvolumes would even be optional):
/dev/sda1 ext3 /boot
/dev/sda2 btrfs / *"menuentry" in /boot/grub/grub.cfg > title "Arch Linux - SB0"
/dev/sda3 btrfs / *"menuentry" in /boot/grub/grub.cfg > title "Arch Linux - SB1"
/dev/sda4 btrfs /home
Basically you have two separate installations, which is what it sounds like you're asking for that can be selected at boot time and share a /home partition.  The fstabs in each root would only differ at the / partition entry.