Cannot connect to the Docker daemon. Is the docker daemon running on this host?

A user raised a ticket to increase /var filesystem because he needed more space for docker images. We normally don’t add space to /var as it is considered a system filesystem. If application needs to write to a directory under /var, we create a new filesystem mounted at for example /var/application-dir.

As docker store images etc. under /var/lib/docker, we created a new filesystem mounted at /var/lib/docker. The user had no images to migrate, therefore we just cleared /var/lib/docker before mounting the new filesystem. After this, docker did not work. systemctl status docker shows docker is up and loaded but running any “docker command” ends up in:

Cannot connect to the Docker daemon. Is the docker daemon running on this host?

We reinstalled docker a few times without success. In the midst of troubleshooting, I killed all processes except my login shell by running killall5. At which point I have no choice apart from reboot. The issue went away after reboot.

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