Creating and using an initrd for Firecracker

Creating

Based on alpine or suse

You can use the script found here to generate an initrd either based on alpine or suse linux.

The script extracts the init system from each distribution and creates a initrd.

Custom

Use this option for creating an initrd if you're building your own init or if you need any specific files / logic in your initrd.

bash

mkdir initrd
cp /path/to/your/init initrd/init
# copy everything else you need in initrd/
cd initrd
find . -print0 | cpio --null --create --verbose --format=newc > initrd.cpio

Usage

When setting your boot source, add a initrd_path property like so:

shell

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/boot-source'   \
    -H 'Accept: application/json'           \
    -H 'Content-Type: application/json'     \
    -d "{
        \"kernel_image_path\": \"/path/to/kernel\",
        \"boot_args\": \"console=ttyS0 reboot=k panic=1 pci=off\",
        \"initrd_path\": \"/path/to/initrd.cpio\"
    }"

Notes

You should not use a drive with is_root_device: true when using an initrd
Make sure your kernel configuration has CONFIG_BLK_DEV_INITRD=y
If you don't want to place your init at the root of your initrd, you can add rdinit=/path/to/init to your boot_args property
If you intend to pivot_root in your init, it won't be possible because the initrd is mounted as a rootfs and cannot be unmounted. You will need to use switch_root instead.