This weekend was my first adventure in Snaps. Unfortunately, this adventure quickly devolved into a murder mystery, for my hard drive. The first snap I try to compile triggers a “disk is almost out of space” notification?! Turns out building Snaps requires a surprising amount of disk space. But it wasn’t entirely Multipass’ fault, my good old friend Docker was eating up a decent chunk of my disk too.

If you are like me, you have a separate (larger) data partion and probably want the Docker and Multipass data directories to live there instead of your small OS root partition.

Getting my laptop

Earlier this year we got my wife a new computer. Originally, it was supposed to be a simple “web and netflix” machine. However, being American expats in Germany, the “market” had a different plan for us. Most budget computers in Germany only come with German keyboard layouts. Very natural, no real complaint. But this means we had to buy something at the professional level. We choose a Dell XPS 13.

The bad news is that my wife ultimately didn’t like the laptop. Primarily, she didn’t liek the keyboard. The good news is that it means I got a new laptop this year.

Setting up my laptop

For the past 10 years I have been a Mac user due it being offered at work. But I have been itching to get back to Linux. So I installed Ubuntu 20.20 :)

In the past I have always used 2 or 3 partitions, one for the OS, one for data, and optionally one for a second OS. On the XPs I have a separate /home mount which contains almost all of my disk space 230GB and a root / partition with just 37GB.

I thought this would be enough space for the OS, until I tried to compile a snap. 5 mins later and a notification pops up “You are almost out of disk space, 1 GB remaining”. Now this is shocking since I don’t remember installing any massive pieces of software.

After several iterations of df -h, du -h, lsof, etc. I pin point the problem to: Docker and Multipass saving every image, vm, and data mount to my /var folder. After recovering my disk space, I set out to move the data locations for docker and multipass to my /home partition, which has plenty of space.

Using the data partition

Setting up docker

Docker is the easy one, it has a configuration specifically for this use case.

  1. First I cleaned up the existing Docker data,
docker system prune -f --volumes --all

You can probably skip this, but I prefer fresh starts.

  1. Then, stop the docker daemon:
sudo service docker stop
  1. Next, create your new data folder
sudo mkdir -p /home/docker/data

the folder will be owned by root, but this is ok.

  1. Now add or edit the configuration file /etc/docker/daemon.json so that it contains
{
  "data-root": "/home/docker/data"
}
  1. Now you can copy and remaining Docker metadata to the new location
sudo rsync -aP /var/lib/docker/ /home/docker/data

If you are being cautious, rename the old data folder so that you can safely revert if anything goes wrong

sudo mv /var/lib/docker /var/lib/docker.old
  1. Resart the docker daemon and test everything is work
sudo service docker start
docker run --rm hello-world
  1. Cleanup the old data
sudo rm -rf /var/lib/docker.old

Setting up multipass

Multipass is a bit harder than Docker. It can be done, but the feature is much newer. It was just merged in October 2020

The author Michał has kindly (provided instructions)[https://github.com/canonical/multipass/pull/1789#issuecomment-704991752] that I have adapted and extended below

  1. First I want to safely clear any cached multipass data, the simplest way to do this is the uninstall and reinstall multipass
sudo snap remove multipass
sudo snap install multipass
  1. Make the new data directory
sudo mkdir /home/multipass
  1. I created a new group for my user and root, so that I would have simpler access to the data directory
sudo groupadd multipass
sudo usermod -a G root multipass
sudo usermod -a -G multipass $USER
sudo chown -R root:multipass /home/multipass
  1. Stop you multipass daemon and enable the required volumn plugins for snap
sudo snap stop multipass
sudo snap connect multipass:removable-media  # for /mnt /media https://snapcraft.io/docs/removable-media-interface
sudo snap connect multipass:all-home  # for /home/* https://snapcraft.io/docs/home-interface
  1. Update the multipass service definition to set the required MULTIPASS_STORAGE environment variable.
sudo mkdir /etc/systemd/system/snap.multipass.multipassd.service.d/
sudo tee /etc/systemd/system/snap.multipass.multipassd.service.d/override.conf <<EOF
[Service]
Environment=MULTIPASS_STORAGE=/home/multipass
EOF
  1. Reload the service defitions
sudo  systemctl daemon-reload
  1. Restart the multipass daemon
sudo snap start multipass
  1. Check that things are working
$ ls -al /home/multipass
total 16K
drwxrwxr-x 4 root multipass 4,0K Dez 13 16:00 .
drwxr-xr-x 6 root root      4,0K Dez 12 21:58 ..
drwxr-xr-x 3 root root      4,0K Dez 13 16:00 cache
drwxr-xr-x 6 root root      4,0K Dez 13 16:00 data

If you start a new vm, e.g. using multipass shell, then you will start seeing data in the /home/multipass/cache/vault/images, for example.

A networking hiccup

Doing all of that, I now have a workign Docker and (I thought) a working Multipass with images and other data stored in my data partition. But this is a premature victory.

Within the last couple days, it seems that Docker changed the way it uses iptables. The consequence is that the iptables Multipass needs are disabled and the VMs have no network access. See this Github issue.

Networking questions are generally above my paygrade, but the summary seems to be that the Multipass snap uses legacy iptables while Docker switched to nftables. Once nftables are used, the legacy tables are ignored.

This looks like

$ sudo iptables -S
# Warning: iptables-legacy tables present, use iptables-legacy to see them
-P INPUT ACCEPT
-P FORWARD DROP
-P OUTPUT ACCEPT
-N DOCKER
-N DOCKER-ISOLATION-STAGE-1
-N DOCKER-USER
-N DOCKER-ISOLATION-STAGE-2
-A FORWARD -j DOCKER-USER
-A FORWARD -j DOCKER-ISOLATION-STAGE-1
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -o docker0 -j DOCKER
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2
-A DOCKER-ISOLATION-STAGE-1 -j RETURN
-A DOCKER-USER -j RETURN
-A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP
-A DOCKER-ISOLATION-STAGE-2 -j RETURN

The solution is to update the iptables/nftables. At this time, I am still researching how to do this without breaking my laptop. If you already know iptables and nftables, please let me know on Twitter


programming multipass docker