Ubuntu – Docker service starts before ZFS

dockerUbuntuubuntu-16.04zfszfsonlinux

I am using Docker CE on Ubuntu 16.04, with ZFS as storage for Docker. Setup is pretty much standard:

  1. There's a zpool that has multiple disks
  2. On the zpool, there's a zfs filesystem with mountpoint /var/lib/docker.
  3. Docker sees that the filesystem is ZFS, and uses that automatically

Everything works most of the time. However, once in a while when my VM boots up, the zpool fails to mount. I believe (though I am not 100% sure) that this is because the Docker service starts before the ZFS filesystem is mounted, and in fact I see a /var/lib/docker folder created on the root FS.

How can I ensure that the Docker service does not start until all ZFS filesystems are mounted?

Best Answer

I was able to solve this by doing two things. Note that one alone might be sufficient.

First, explicitly tell Docker to use ZFS as filesystem, by writing {"storage-driver": "zfs"} in the file /etc/docker/daemon.json. (If the file exists in your disk, then just add the storage-driver key)

Second, create the following systemd unit in the file /etc/systemd/system/docker-wait-zfs.service:

[Unit]
Description=Wait for ZFS before starting Docker
RequiredBy=docker.service
Before=docker.service
Requires=zfs.target
After=zfs.target

[Service]
Type=oneshot
ExecStart=/bin/true
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Then execute:

systemctl enable docker-wait-zfs.service
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