Advanced topic – changing the default logging driver

Let's change the default logging driver of a Linux host. The easiest way to do this is on a real Linux host. For this purpose, we're going to use Vagrant with an Ubuntu image:

$ vagrant init bento/ubuntu-17.04
$ vagrant up
$ vagrant ssh

Once inside the Ubuntu VM, we want to edit the Docker daemon configuration file. Navigate to the folder /etc/docker and run vi as follows:

$ vi daemon.json  

Enter the following content:

{
  "Log-driver": "json-log",
  "log-opts": {
    "max-size": "10m",
    "max-file": 3
  }
}

Save and exit Vi by first pressing Esc and then typing :w:q  and finally hitting the ENTER key.

The preceding definition tells the Docker daemon to use the json-log driver with a maximum log file size of 10 MB before it is rolled, and the maximum number of log files that can be present on the system is 3 before the oldest file gets purged.

Now we have to send a SIGHUP signal to the Docker daemon so that it picks up the changes in the configuration file:

$ sudo kill -SIGHUP $(pidof dockerd) 

Note that the preceding command only reloads the config file and does not restart the daemon.