Converting rootfs to BTRFS

Note: Converting rootfs to BTRFS via btrfs-convert does not work at this time. System succesfully boots, however an exception is thrown by the kernel and rootfs is re-mounted readonly.

Note2: You don't have to rent a temporary disk volume, you may use a storage over network if you like. Adapt the below procedure accordingly if you prefer to do so.

1. Rent a second disk volume.

2. Boot in Rescue Mode. Finnix OS will be launched in a few seconds.

   1. Be sure you attached the newly rent disk before booting in rescue mode

3. Backup your rootfs:

    mkfs.btrfs /dev/sdc 
    mkdir /mnt/a /mnt/c
    mount /dev/sda /mnt/a
    mount /dev/sdc /mnt/c
    rsync -avP /mnt/a/ /mnt/c/
   

4. Format the rootfs disk:

    umount /dev/sda
    mkfs.btrfs /dev/sda
    mount /dev/sda /mnt/a
   

5. Copy your rootfs files back to /dev/sda

    rsync -avP /mnt/c/ /mnt/a/
   

6. Edit /mnt/a/etc/fstab accordingly to change rootfs type to BTRFS:

    /dev/sda       /               btrfs    defaults          0       1
   

7. Use the UUID value of the following command in the next step:

    blkid | grep /dev/sda
   

8. Edit /mnt/a/boot/grub/grub.cfg to replace UUID of /dev/sda in the menuentries accordingly

9. Reboot the system

10. If you successfully booted, update grub.cfg properly:

    update-grub
    

11. Do not forget to install btrfs-tools:

    apt-get install btrfs-tools
    

Move / to a subvolume (rootfs)

1. btrfs sub snap / /rootfs

2. Create the new top subvolume mountpoint:

    mkdir /rootfs/mnt/myhost
   

3. Edit the /etc/fstab now (to save one reboot):

    /dev/sda  /             btrfs   subvol=/rootfs,rw,noatime 0 1
    /dev/sda  /mnt/myhost   btrfs   subvolid=5,noatime  0 1
   

4. Edit the linux ... lines in /boot/grub/grub.cfg accordingly to mount /rootfs subvolume at boot time:

    ... rootflags=subvol=rootfs ... 
   

5. Reboot

6. Check if everything went fine so far:

   1. Check if there is any BTRFS error:

       # cat /var/log/messages | grep -i btrfs | grep -i error
      

   2. Check that our new / is the rootfs subvolume:

       # btrfs subvolume show /
               Name:                   rootfs
               ...
      

7. If everything went fine, update grub to reflect the appropriate changes:

      ### do not forget to backup your /boot/grub/grub.cfg first 
      # update-grub 
   

8. Reboot

9. If everything still okay, cleanup the old files:

      # find /mnt/myhost -maxdepth 1 -mindepth 1 -not -name rootfs -exec rm -r {} \;
   

10. Fix your Grub configfile location:

    You can load your configfile inside Grub Rescue Shell by the following command:

      configfile (hd0)/rootfs/boot/grub/grub.cfg
    

    1. btrfs sub create /mnt/myhost/boot

    2. mv /boot/* /mnt/myhost/boot/

    3. Add the following line to /etc/fstab to mount /boot from /mnt/myhost/boot:

       /dev/sda       /boot           btrfs    subvol=/boot,rw,noatime       0     1
       

    4. update-grub