TIL: Systemd Journald Cannot Clear the Logs of a Specific Unit#
Some logs of my Docker containers got out of hand, so I tried Docker’s Journald logging driver. It did not take long for the log of the service to grow again, but no problem. Here’s the plan:
reconfigure service to be less verbose
truncate logs
€€€
… or so I thought.
As always, systemd does not disappoint: It’s impossible to remove the log of a single unit using its journalctl command: https://stackoverflow.com/a/37915789
Of course, there are workarounds, but I don’t want workarounds. I want sane, “standard” handling of things.
Well… Back to files, a UNIX concept that proved flexible and powerful since the dawn of time [1].
cat <<'EOF' > /etc/docker/daemon.json
{
"log-driver": "json-file",
"log-opts": {
"max-size": "200m",
"max-file": "1"
}
}
EOF
systemctl restart docker.service
Note that running containers have to be stopped, removed and restarted for them to use the new log driver. Afterwards you find the log files here:
ls /var/lib/docker/containers/*/*json.log
For log aggregation and storage, I use promtail. I have to use the JSON File logging driver, because Promtail’s docker_sd_config demands that “The containers must run with either the json-file or journald logging driver.”. I would like to use the Local File logging driver, because on the machine itself, I prefer performance and minimal disk use.