Btrfs

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. For an introduction, see Wikipedia article and/or the dedicated wiki.

WARNING: this is EXPERIMENTAL and could cause DATA LOSS.

This was updated on 2020-09-24 for Debian 10 buster + GandiV5, and on 2022-02-02 for Debian 11 bullseye + GandiV5. 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 or Debian 11 bullseye.

Update the packages:

apt update && apt upgrade -y

For Debian 10 buster (only), define backports (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

Install btrfs-progs:

apt update && apt install -t buster-backports btrfs-progs # Debian 10 buster
apt update && apt install btrfs-progs # Debian 11 bullseye

On GandiV5 :

1. stop the server,
2. open the page about the system disk,
3. clone the system disk with another (definitive) name,
4. start the server,
5. attach this cloned disk to the server.

ATTENTION: really start the server before attaching the cloned disk, else the root filesystem might be the cloned disk given they have the same UUID and /etc/fstab is using the UUID to select the root filesystem (it could be workarounded by setting /dev/xvda1 in /etc/fstab before stopin the server).

Convert to btrfs

Mostly from [1]

Display the active root filesystem, and you will be able to deduce what is the cloned filesystem:

ls /dev/xvd*
mount|grep xvd
blkid

Let’s say /dev/xvdb1 is the clone filesystem, we convert it to btrfs: (man 8 btrfs-convert)

fsck.ext4 -f /dev/xvdb1 # optional
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

According to [2] and [3], it can be worked around: (this command could take some hours, depending on disk size and number of files)

btrfs-convert -d -p /dev/xvdb1

or even:

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 [4]

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

There are 3 things to change on this line: UUID, ext4 → btrfs, and the options now to "defaults".

(in vi, type "dd" to remove a line, "i" to enter in edit mode, Esc to quit edit mode, ":x" to save and quit.)

Update grub:

update-grub

Change the reference of the root device (will be xvda1 when it will be the root device):

sed -i s/xvdb1/xvda1/g /boot/grub/grub.cfg

Quit:

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)).

df -hT

It should show something like:

/dev/xvda1     btrfs      50G   32G   19G  63% /

Delete the old root disk.

You can delete the packages grub-efi-amd64, grub-efi-amd64-bin, grub-pc, grub-pc-bin since they are not used, but keep /etc/default/grub with Gandi customisations:

cp -a /etc/default/grub /root/grub
apt-get purge -y grub-efi-amd64 grub-efi-amd64-bin grub-pc grub-pc-bin
mv /root/grub /etc/default/grub
apt-mark manual grub-common grub2-common
update-grub # to check it still work and you should reboot now to be sure it reboots correctly

Troubleshootings

• During a conversion, the root filesystem was mounted read-only in btrfs; it was probably because I didn’t change the options "rw,noatime,errors=remount-ro" to "defaults"

External links

• PvGrub2 on Xen wiki