Btrfs in RAID1 as a root filesystem on Gentoo

-README.md

Btrfs in RAID1 as a root filesystem on Gentoo

Partitioning scheme

Partition	Filesystem	Size	Description
sd*1	ext2 (md/raid1)	256 MiB	boot (kernel etc.)
sd*2	sw (md/raid1)	4 GiB	swap
sd*3	Btrfs (raid1)	*	Btrfs

A. Create GPT partitions and filesystems

1. Create GPT partition table on each disk:

   parted -a optimal
   unit MiB
   mklabel gpt
   mkpart primary 1 256
   mkpart primary 256 4352
   mkpart primary btrfs 4352 -1
   set 1 legacy_boot on
   quit
   

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

   mkfs.btrfs -L btrfsvol -d raid1 -m raid1 /dev/sda3 /dev/sdb3
   mkdir /mnt/btrfs
   mount -t btrfs -o defaults,noatime,compress=lzo,autodefrag -L btrfsvol /mnt/btrfs
   btrfs subvolume create /mnt/btrfs/root
   umount /mnt/btrfs
   mount -t btrfs -o defaults,noatime,compress=lzo,autodefrag,subvol=root -L btrfsvol /mnt/gentoo

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

   mdadm --create /dev/md0 --name boot --level 1 --metadata 1.0 --raid-devices=2 /dev/sda1 /dev/sdb1
   mkfs.ext2 -L boot /dev/md0
   mkdir -p /mnt/gentoo/boot
   mount /dev/md0 /mnt/gentoo/boot

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

4. 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 /dev/sdb2
   mkswap -L swap /dev/md1

B. Install Gentoo

Read handbook ;)

C. Create initramfs

1. Install busybox and mdadm with USE="static", and btrfs-progs:

   USE="static -pam" emerge -av sys-apps/busybox
   USE="static" emerge -av sys-fs/mdadm
   emerge -av sys-fs/btrfs-progs

2. Create directory /usr/src/initramfs and copy files initramfs_list and init into it.

3. Generate mdadm.conf:

   mdadm --detail --scan > /usr/src/initramfs/mdadm.conf

4. Build initramfs:

   cd /usr/src/linux
   make -C /usr/src/linux/usr/ gen_init_cpio
   ./scripts/gen_initramfs_list.sh -o /boot/initrd.cpio.gz /usr/src/initramfs/initramfs_list

D. Install syslinux bootloader

1. Install syslinux package:

   emerge -va sys-boot/syslinux

2. Copy extlinux MBR for GPT to physical disks (not partions!):

   cat /usr/share/syslinux/gptmbr.bin > /dev/sda
   cat /usr/share/syslinux/gptmbr.bin > /dev/sdb

3. Install extlinux to /boot/extlinux:

   mkdir -p /boot/extlinux
   extlinux --raid --install /boot/extlinux

4. Copy provided extlinux.conf to /boot/extlinux/ and modify path of the kernel file if needed.

extlinux.conf

default linux
# Always display boot prompt.
prompt 1
# Wait 5 seconds unless the user types something.
timeout 50

label linux
    kernel /current-kernel
    initrd /initrd.cpio.gz
    append root=LABEL=btrfsvol rootfstype=btrfs rootflags=defaults,noatime,compress=lzo,autodefrag,subvol=root net.ifnames=0

fstab

# /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>
LABEL="boot"        /boot           ext2        noauto,noatime                                          1 2
LABEL="btrfsvol"    /               brtfs       defaults,noatime,compress=lzo,autodefrag,subvol=root    0 1
LABEL="swap"        none            swap        sw                                                      0 0

init

#!/bin/busybox sh
 
ROOT_MOUNT_LABEL="btrfsvol"
ROOT_MOUNT_OPTS="defaults,noatime,compress=lzo,autodefrag,subvol=root"
SWAP_MOUNT_LABEL="swap"

rescue_shell() {
    echo "$@"
    echo "Something went wrong. Dropping you to a shell."
    busybox --install -s
    exec /bin/sh
}
 
mount_root() {
    echo "Mounting root filesystem"

    /sbin/btrfs device scan || return 1
    mount -t btrfs -o $ROOT_MOUNT_OPTS \
        $(dev_by_label "$ROOT_MOUNT_LABEL") /mnt/root
}

mount_swap() {
    echo "Assembling RAID arrays and enabling swap"

    /sbin/mdadm --assemble --scan || return 1
    swapon $(dev_by_label "$SWAP_MOUNT_LABEL")
}

dev_by_label() {
    findfs LABEL="$1" || {
        echo "FS with LABEL=$1 not found"
        return 1
    }
}

# Temporarily mount /proc, /sys and /dev filesystems.
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs none /dev
 
# Disable kernel messages from popping onto the screen.
echo 0 > /proc/sys/kernel/printk

clear

mount_root || rescue_shell "Failed to mount root filesystem!"
mount_swap || echo "Failed to mount swap. This is not critical, so keep going."
 
echo "All done, switching to real root."
 
# Clean up; the init process will remount them later.
umount /proc
umount /sys
umount /dev
 
# Switch to the real root and execute init.
exec switch_root /mnt/root /sbin/init

initramfs_list

# directory structure
dir /bin        755 0 0
dir /dev        755 0 0
dir /etc        755 0 0
dir /lib64      755 0 0
dir /mnt        755 0 0
dir /mnt/root   755 0 0
dir /proc       755 0 0
dir /root       700 0 0
dir /sbin       755 0 0
dir /sys        755 0 0
dir /usr        755 0 0
dir /usr/lib64  755 0 0
dir /var        755 0 0
 
# busybox (compiled with USE="static")
file /bin/busybox                   /bin/busybox                755 0 0
 
# mdadm (compiled with USE="static")
file /sbin/mdadm                    /sbin/mdadm                 755 0 0

# btrfs-progs
file /sbin/btrfs                    /sbin/btrfs                 755 0 0
file /sbin/btrfs-debug-tree         /sbin/btrfs-debug-tree      755 0 0
file /sbin/btrfs-find-root          /sbin/btrfs-find-root       755 0 0
file /sbin/btrfs-image              /sbin/btrfs-image           755 0 0
file /sbin/btrfs-map-logical        /sbin/btrfs-map-logical     755 0 0
file /sbin/btrfs-show-super         /sbin/btrfs-show-super      755 0 0
file /sbin/btrfs-zero-log           /sbin/btrfs-zero-log        755 0 0
file /sbin/btrfsck                  /sbin/btrfsck               755 0 0
file /sbin/btrfstune                /sbin/btrfstune             755 0 0
file /sbin/fsck.btrfs               /sbin/fsck.btrfs            755 0 0

# btrfs-progs dependencies
file /lib64/ld-linux-x86-64.so.2    /lib64/ld-linux-x86-64.so.2 755 0 0
file /lib64/libblkid.so.1           /lib64/libblkid.so.1        755 0 0
file /lib64/libc.so.6               /lib64/libc.so.6            755 0 0
file /lib64/libpthread.so.0         /lib64/libpthread.so.0      755 0 0
file /lib64/libuuid.so.1            /lib64/libuuid.so.1         755 0 0
file /lib64/libz.so.1               /lib64/libz.so.1            755 0 0
file /usr/lib64/liblzo2.so.2        /usr/lib64/liblzo2.so.2     755 0 0

# init script
file /init                          /usr/src/initramfs/init     755 0 0

# config for md RAID
file /etc/mdadm.conf            /usr/src/initramfs/mdadm.conf   644 0 0