I struggled to get that working, so i thought it would be worth a quick recipe.

I’m not talking about raid1 on disk with mdadm to create mirrored Physical Volume, but rather just use the mirror feature of LVM directly, for convenience.

Overview
         +-------------------------------+
         |       Filestystem  ext4       |
         |       (archlinux OS)          |
         +-------------------------------+
     +-+ +-------------------------------+
     |   | Logical volume with 1 mirror  |
     |   | (type raid1)                  |
     |   +-------------------------------+
LVM  |
     |     +--+    +--+   +--+
     |     |LE|    |LE|   |LE|   ...
     |     ++-+    +--+   +--+
     +--+   | |    each LE maps to 2 PE
            | +------+
            |        |
          +-v--+  +--v-+ +----+ +----+
          |    |  |    | |    | |    |
          |disk|  |disk| |disk| |disk|
          |    |  |    | |    | |    |
          |    |  |    | |    | |    |
          +----+  +----+ +----+ +----+

  1. Convert the LV to mirrored LV

    lvconvert -m1 vg1/archlinux

  2. Watch progress of the synchronization using lvs (Cpy%sync column). You can also display the Sub LVs and physical volumes used as follow:

    # lvs -a -o +devices |grep archlinux
  LV                   VG  Attr       LSize   Cpy%Sync Devices
  archlinux            vg1 rwi-aor--- 120.00g 100.00   archlinux_rimage_0(0),archlinux_rimage_1(0)
  [archlinux_rimage_0] vg1 iwi-aor--- 120.00g          /dev/sdc2(33053)
  [archlinux_rimage_1] vg1 iwi-aor--- 120.00g          /dev/sda1(1)
  [archlinux_rmeta_0]  vg1 ewi-aor---   4.00m          /dev/sdc2(33052)
  [archlinux_rmeta_1]  vg1 ewi-aor---   4.00m          /dev/sda1(0)

  3. Edit /etc/mkinitcpio.conf check MODULES and HOOKS

    MODULES=(ext4 amdgpu raid1 dm_raid)
HOOKS=(base udev autodetect modconf block lvm2 mdadm_udev filesystems keyboard fsck)

  4. run mkinitcpio -p linux

  5. If like me you’re using GRUB, add the appropriate insmod to the menuentry:

    menuentry 'archlinux' --class archlinux --class gnu-linux --class gnu --class os $menuentry_id_option 'archlinux' {
        insmod gzio
        insmod part_gpt
        insmod diskfilter
        insmod lvm
        insmod mdraid1x
        insmod ext2
        set root=(lvm/vg1-archlinux)
        linux /boot/vmlinuz-linux root=/dev/mapper/vg1-archlinux
        initrd /boot/initramfs-linux.img
}

links