initramfs for Btrfs

This is a simple initramfs for booting Linux from a root filesystem on a multi-device, i.e. mirrored Btrfs volume. It is meant to be included in the kernel image.

Setup

• Download init and initramfs.list into the directory /usr/src/sys/initramfs.btrfs
• Adjust your kernel source configuration under General setup select Initial RAM filesystem and RAM disk (initrams/initrd) support
• Enter /usr/src/sys/initramfs.btrfs/initramfs.list as Initramfs source file(s):
  
• Exit and save your kernel configuration
• (Re-)Build your kernel, e.g.:

  $ make -j8 bzImage
  $ make modules

• Install the kernel:

  $ sudo cp arch/`uname -m`/boot/bzImage /boot/vmlinuz
  $ sudo make modules_install

• Your /boot/grub/grub.cfg may be very simplistic, e.g. if the label of your Btrfs root volume is BUTTER (check the output of btrfs fi show):

  set timeout=5
  set root=(hd0,gpt1),(hd1,gpt1)
  
  insmod part_gpt
  insmod btrfs
  insmod linux
  
  menuentry "Linux" {
    linux /boot/vmlinuz root=LABEL=BUTTER
  }

• You may add rootflags=... followed by comma-separated mount options for the root filesystem. Defaults to ro,noatime,nodiratime
• If you add dorescue to the kernel commandline, you will be dropped to a rescue shell inside the initramfs