I tried to have a root filesystem into btrfs for a Gandi server, and succeed after a number of trial-error. Here are my steps if it can be useful to others.
WARNING: this is EXPERIMENTAL and could cause DATA LOSS.
This was updated for Debian 10 buster + GandiV5 and works probably also for Debian 11 bullseye. An older version was for Debian 9 jessie or Ubuntu 18.04 LTS + GandiV4.
Create the server
On GandiV5, create a new server with e.g. 1 proc, 256 Mio RAM, 1 system disk, Debian 10 buster.
Update the packages:
apt update && apt upgrade -y
Install grub-xen instead of grub-efi-amd64:
cp -a /etc/default/grub /root/grub apt purge grub-efi-amd64-bin && apt install grub-xen mv /root/grub /etc/default/grub
Define backports for Debian 10 (btrfs-progs must contain btrfs-convert, see this Debian bug) (this is not needed for Debian 11):
echo 'deb https://deb.debian.org/debian buster-backports main' > /etc/apt/sources.list.d/debian-backports.list
apt update && apt install -t buster-backports btrfs-progs
On GandiV5, stop the server, open the page about the system disk, clone the system disk with another (definitive) name, and attach this cloned disk to the server, start the server.
Convert to btrfs
- Mostly from 
Display the active root filesystem, and you will be able to deduce what is the cloned filesystem:
ls /dev/xvd* mount|grep xvd blkid -o value -s TYPE /dev/xvdb1 # and the same for others
Let’s say /dev/xvdb1 is the clone filesystem, we convert it to btrfs: (man 8 btrfs-convert)
fsck.ext4 -f /dev/xvdb1 btrfs-convert -p /dev/xvdb1
Here, I obtained the following error:
root@test:~# btrfs-convert -p /dev/xvdb1 create btrfs filesystem: blocksize: 4096 nodesize: 16384 features: extref, skinny-metadata (default) creating ext2 image file creating btrfs metadata Unable to find block group for 0 27081] Unable to find block group for 0 Unable to find block group for 0 ctree.c:2245: split_leaf: BUG_ON `1` triggered, value 1 btrfs-convert(+0x11b5a)[0x559c159c1b5a] btrfs-convert(+0x1589b)[0x559c159c589b] btrfs-convert(btrfs_search_slot+0x269)[0x559c159c6401] btrfs-convert(btrfs_insert_empty_items+0x92)[0x559c159c7b3c] btrfs-convert(btrfs_record_file_extent+0x1bc)[0x559c159d46b4] btrfs-convert(record_file_blocks+0x14a)[0x559c159bfe92] btrfs-convert(+0x10349)[0x559c159c0349] btrfs-convert(+0x1135f)[0x559c159c135f] btrfs-convert(main+0x1f59)[0x559c159bdefb] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f4c11c3bb97] btrfs-convert(_start+0x2a)[0x559c159bb5ca] Aborted
btrfs-convert -d -p /dev/xvdb1
btrfs-convert -n -d -p /dev/xvdb1
Or, if still unsuccessful, try to add free space, or if still unsuccessfull, re-compile btrfs-progs in version 4.17.1 (this last try worked for me for 50 Gio disk with 24 Gio free space on Debian 9 jessie, expanded after its original size was 30 Gio (=4 Gio free space)).
Then, mount the filesystem and delete the old ext4 snapshot:
mount /dev/xvdb1 /mnt btrfs subvolume delete /mnt/ext2_saved btrfs filesystem defrag -r /mnt # could take dozen of minuts btrfs balance start /mnt # could take hours
Promote as root filesystem
- Mostly from 
Enter the chrooted system:
for i in dev dev/pts proc sys; do mount --bind /$i /mnt/$i; done chroot /mnt blkid|grep xvdb1 vi /etc/fstab
Edit the root filesystem with: (the UUID is from the command blkid)
UUID=a74f5787-aee1-4981-b7e6-fbd3cb6ac919 / btrfs defaults 0 1
(in vi, type "dd" to remove a line, "i" to enter in edit mode, Esc to quit edit mode, ":x" to save and quit.)
grub-install --target x86_64-xen /dev/xvdb update-grub
exit for i in dev/pts dev proc sys; do umount /mnt/$i; done umount /mnt exit
Define the cooked disk as boot disk
In Gandi V5 interface, stop the server, detach the old root disk and define the cooked disk as "Use to start".
Launch the server (it should correctly start, even if the /boot directory is on the main partition / (some old documents said it didn’t work because grub didn’t know the btrfs filesystem, but it is fixed now)).
It should show something like:
/dev/xvda1 btrfs 50G 32G 19G 63% /
Delete the old root disk.