Btrfs in RAID1 as a root filesystem on Debian

Migrating Debian from a single disk with ext4 to two disks with btrfs and RAID1.

Source: https://gist.github.com/jirutka/990d25662e729669b3ce (for Gentoo and syslinux)

Partitioning scheme

Partition	Filesystem	Size	Description
sd*1	ext2 (md/raid1)	512 MiB	boot (kernel etc.)
sd*2	sw (md/raid1)	4 GiB	swap
sd*3	Btrfs (raid1)	*	Btrfs

A. Create GPT partitions and filesystems

Create GPT partition table on each disk:

parted -a optimal /dev/<disk>
unit MB
mklabel gpt
mkpart primary 1 512
mkpart primary 512 4600
mkpart primary 4600 4608
mkpart primary btrfs 4608 -1
set 1 legacy_boot on
set 3 bios_grub on
print
quit

Create Btrfs filesystem in raid1 mode on the third partitions, create subvolume for / and mount it into /mnt/debian:

When you have two empty disks:

```sh
mkfs.btrfs -L btrfsvol -d raid1 -m raid1 /dev/sda3 /dev/sdb3
```

When you migrate from one to two disks and the first is still in use (which is assumed in this tutorial):

```sh
mkfs.btrfs -L btrfsvol /dev/sdb3
```

and later when you are finished add the second disk and create the RAID1 (See step E):

```sh
btrfs device add /dev/sda3 /
btrfs balance start -dconvert=raid1 -mconvert=raid1 /
```

```sh
mkdir /mnt/btrfs
mount -t btrfs -o defaults,noatime,compress=no,autodefrag -L btrfsvol /mnt/btrfs
btrfs quota enable /mnt/btrfs/
btrfs subvolume create /mnt/btrfs/root
btrfs subvolume create /mnt/btrfs/var
umount /mnt/btrfs
mkdir /mnt/debian
mount -t btrfs -o defaults,noatime,compress=no,autodefrag,subvol=root -L btrfsvol /mnt/debian
mkdir /mnt/debian/var
mount -t btrfs -o defaults,noatime,compress=no,autodefrag,subvol=var -L btrfsvol /mnt/debian/var
btrfs qgroup limit 21G /mnt/debian/var/
```

Create md raid1 array with ext2 filesystem on the first partitions and mount it to /mnt/debian/boot:

mdadm --create /dev/md0 --name boot --level 1 --metadata 1.0 --raid-devices=2 /dev/sda1 missing
mkfs.ext2 -L boot /dev/md/boot_0
mkdir -p /mnt/debian/boot
mount /dev/md/boot_0 /mnt/debian/boot

Note: extlinux cannot boot from md with metadata v1.2 (read here)!

Create md raid1 array with swap on the second partitions:

mdadm --create /dev/md1 --name swap --level 1 --metadata 1.2 --raid-devices=2 /dev/sda2 missing
mkswap -L swap /dev/md1

B. Move Debian

Copy existing Debian installation to new (degraded) RAID1:

```sh
cp -rpa /bin /boot /etc /initrd.img* /lib* /root /sbin /srv /usr /var /vmlinuz* /mnt/debian/
cd /mnt/debian/
mkdir dev
mkdir proc
mkdir run
mkdir sys
mkdir media
mkdir mnt
mkdir tmp
chmod 777 tmp/
chmod +t tmp/
```

mkdir dev && mkdir proc && mkdir run && mkdir sys && mkdir media && mkdir mnt && mkdir tmp

C. Adjust fstab and Grub config

Get UUIDs of all newly created partitions

The UUID (check with blkid) is the same for both partitions in our RAID, so no need to specify devices. sh blkid --match-token TYPE=btrfs blkid --match-token TYPE=swap blkid --match-token LABEL=boot

Edit fstab
```
nano /mnt/debian/etc/fstab
```

We also add "degraded" as file system option in fstab; e.g.: ``` # /etc/fstab: static file system information. # # The root filesystem should have a pass number of either 0 or 1. # All other filesystems should have a pass number of 0 or greater than 1. # # See the manpage fstab(5) for more information. #

# <fs>              <mountpoint>    <type>      <opts>                                              <dump/pass>
UUID=3cd3dc71-091c-9c90-4b7c-bd46a90c4964    /boot           ext2        noauto,noatime                                          1 2
UUID=48d773c8-f3d5-40e9-9a2c-ab1f68195a63    /               brtfs       defaults,noatime,degraded,compress=no,autodefrag,subvol=root    0 1
UUID=48d773c8-f3d5-40e9-9a2c-ab1f68195a63    /var            brtfs       defaults,noatime,degraded,compress=no,autodefrag,subvol=var    0 0
UUID=558903fd-310c-47cc-9ebe-4e42e32990bd    none            swap        sw                                                      0 0
```

Edit Grub config
```
nano /etc/default/grub
```

We also have to add "degraded" as kernel option e.g. (you can only have one rootflags argument, therefore we have to repeat the subvol= option):

```
GRUB_CMDLINE_LINUX_DEFAULT="rootflags=subvol=root,degraded quiet"
```

D. Re-Install Grub bootloader

Basically follow the Debian wiki article to reinstall Grub: https://wiki.debian.org/GrubEFIReinstall

```sh
for i in /dev /dev/pts /proc /sys /sys/firmware/efi/efivars /run; do mount -B $i /mnt/debian/$i; done
chroot /mnt/debian
grub-install /dev/sdb
update-grub
```

Shut-down the computer and disconnect the old disk. We want to make sure we boot from the new (degraded) RAID.

Switch on the computer and test if everything is (still) working as before.

You successfully moved your Debian to a new disk!

E. Format and add old disk

Only continue with this step if everything works! Fix the bugs/copy missing files from the old disk before you format it!

Repeat step A.1 with the old disk where Debian was installed before without RAID.

Then add the newly created partitions to the existing (degraded) RAID 1:

```sh
mdadm /dev/md/boot_0 --add /dev/sdb1
mdadm /dev/md/swap_0 --add /dev/sdb2
btrfs device add /dev/sdb3 /
btrfs balance start -dconvert=raid1 -mconvert=raid1 /
```

You can check the progress of mdadm with: sh cat /proc/mdstat

Then install Grub on the second disk as well: sh grub-install /dev/sdb update-grub

Test your RAID. Shut-down the computer and disconnect one disk. Switch on the computer and test if everything is working. Shut-down and repeat with the other disk.

Hints

How to display quota limit on btrfs subvolume? sh btrfs qgroup show -pcre /

Links

https://seravo.fi/2015/using-raid-btrfs-recovering-broken-disks https://www.complang.tuwien.ac.at/anton/btrfs-raid1.html https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices https://wiki.debian.org/GrubEFIReinstall

phedre/btrfs-raid-boot.md