btrfs-convert is not well tested and recommended. Especially for Kernels > 4.0. Personaly it generates errors on BTRFS which will appear after some time. Better way is to use additional temporary disc to move data to newly created BTRFS.

fsck -f /dev/sda1
fsck -f /dev/sda6
 
btrfs-convert /dev/sda1

Disk usage before conversion: used 1.56GB, free 2.53GB
after conversion: used 1.65GB, free 2.43GB

btrfs check /dev/sda1

Mount new root filesystem:

mount /dev/sda1 /mnt
for i in dev dev/pts proc sys ; do mount --bind /$i /mnt/$i ; done
chroot /mnt

get new UUID of /dev/sda1 using “blkid” mount /dev/sda1 somewhere, go to etc/fstab and update to new UUID:

UUID=a74f5787-aee1-4981-b7e6-fbd3cb6ac919 /               btrfs    defaults 0       1

mount /boot  # if on separate partition
update-grub
grub-install /dev/sda
umount /boot
 
exit  # from chrooted env
 
reboot

and remove Live CD / USB with Gparted

Rollback to ext2 is possible. There is subvolume ext2_saved on filesystem.

btrfs subvolume list /

To delete backup

btrfs subvolume delete /ext2_saved