Performance issue with docker on ubuntu 20.04 LTS

If you installed Docker on Ubuntu 20.04 LTS before version 19.03.8-0ubuntu2, you may have a dataset layout dedicated to docker that is not efficient enough over time and may lead to ZSys timeout, slow boot, disk space limitations and more issues.

New installations after docker.io version 19.03.8-0ubuntu2 are not impacted, but we decided to not migrate existing installations automatically as it requires to stop docker.

This has been discussed on this bug report and directly with Docker upstream.

To migrate manually, you can run this script as root. /!\ It will stop the docker daemon. If you have a specific configuration preventing docker to be stopped, please replace the systemctl commands accordingly.

set -eu
# Check where the closest mountpoint is (if it’s zfs)
docker_parent_mountpoint="$(findmnt -n -o SOURCE -T /var/lib/docker -t zfs || true)"
if [ -n "${docker_parent_mountpoint}" ]; then
    if [ "$(zfs get mountpoint -H -o value "${docker_parent_mountpoint}")" != "/var/lib/docker" ]; then
        pool="${docker_parent_mountpoint%%/*}"

        # Create <currentpool>/var/lib/docker (if it doesn’t exist)
        zfs create -o canmount=off "$pool"/var 2>/dev/null || true
        zfs create -o canmount=off "$pool"/var/lib 2>/dev/null || true
        # Migrate existing content
        systemctl stop docker
        mv /var/lib/docker /var/lib/docker.migrating
        dockermnt="$pool"/var/lib/docker
        zfs create "${dockermnt}" 2>/dev/null || true
        chmod 0711 /var/lib/docker

        # Migrate from older path to newer one.
        mv /var/lib/docker.migrating/* /var/lib/docker/ 2>/dev/null || true
        rmdir /var/lib/docker.migrating/
        # Migrate datasets if they were created by ZSys
        rpool="$(zfs mount | awk '$2=="/" {print $1}'|  cut -d/ -f1)"
        for dataset in $(zfs list -H -t filesystem -o name,mountpoint,canmount -r "$rpool"/ROOT 2>/dev/null | grep -E "/var/lib/[0-9a-f]{64}(-init)?\slegacy\son" | cut -f1); do
            dockerid=${dataset##*/}
            zfs rename "$dataset" "${dockermnt}/$dockerid" || true
        done
        # Purge their ZSys history datasets
        for dataset in $(zfs list -H -t snapshot -o name -r "${dockermnt}" | grep "@autozsys_"); do
            zfs destroy -R "$dataset" || true
        done
        systemctl start docker
    fi
fi