Julia Evans

Day 43: Building VM images

I spent all day yesterday trying to build Ubuntu VM images that work with Firecracker without too much success.

some miscellaneous things I learned about manipulating images

At some point I was trying to extract the filesystem from the Ubuntu cloud image to use with Firecracker. This did not work

  • I can use sfdisk to view the partition table of a disk image
  • I can use dd to extract dd if=focal-server-cloudimg-amd64.img.orig.raw of=focal-image.ext4 skip=227328 count=4384735
  • Some Linux files are “sparse” which means that regions which are filled with 0s are collapsed. Very useful for disk images.
  • I can use fallocate -d to turn a non-sparse file into a sparse file

compiling the linux kernel isn’t that slow

The Firecracker instructions suggest building your own Linux kernel from source. This seemed intimidating to me because I hadn’t done it before but it was actually totally fine!

It only took me like 5 minutes to compile a Linux kernel from scratch (with make -j12 on an AMD Ryzen 5). I was really surprised by this, I thought it would take like 2 hours.

I found out that the Linux kernel I compiled didn’t have the ISO filesystem support, so I needed to reconfigure it to include ISO filesystem support. This was very easy.

the Ubuntu docker image doesn’t seem like the best base for a VM image

I’m still not sure what Ubuntu image I should use as a base for my VMs. I’ve been trying both the Ubuntu cloud image (https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img) and the Docker Ubuntu:20.04 image

Problems with the Ubuntu cloud image: I haven’t been able to get it to boot yet for some reason I haven’t worked out. It gets stuck on this A start job is running for /dev/disk/by-label/UEFI step that Idon’t understand.

Problems with the Docker image: it does boot, but I haven’t managed to convince cloud-init to successfully run in it yet. I also need to install a some basic things in it (like init and udev for some reason), and it’s not really designed to be a system to log into so I feel like it’s missing a lot of things

Maybe today I’ll go back to the cloud image and see what problems there are with getting it to boot.

Day 42: Writing a Go program to manage Firecracker VMs Day 44: Building my VMs with Docker