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I have a very large log file under /var/lib/docker/container/<container_hash>/...-json.log

Is it possible to remove it while the container is still running? Would it create a new one instead and keep writing into that (preferred option)?

CIsForCookies
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2 Answers2

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No : Docker is not designed to re-create the log file if you delete it manually.
A better approach if logs consistency doesn't matter for you would be to clear the content of the log file. You can do it with (sudo is required because the owner/group of docker stuffs is root):

 sudo sh -c "truncate -s 0 /var/lib/docker/container/<container_hash>/...-json.log"

About your question :

Would that be OK even if the file is constantly being written into? –

No error at least. Here a simple Linux example to check that :

Run a container that writes logs every 0.5 second :

docker run -d --name while-true  alpine sh -c "while true; do date; sleep 0.5s; done"

Show 20 last logs :

docker logs --tail=10 while-true
Sun Apr 11 17:29:27 UTC 2021
Sun Apr 11 17:29:27 UTC 2021
Sun Apr 11 17:29:28 UTC 2021
Sun Apr 11 17:29:28 UTC 2021
Sun Apr 11 17:29:29 UTC 2021
Sun Apr 11 17:29:29 UTC 2021
Sun Apr 11 17:29:30 UTC 2021
Sun Apr 11 17:29:30 UTC 2021
Sun Apr 11 17:29:31 UTC 2021
Sun Apr 11 17:29:31 UTC 2021

Truncate the log to 0 byte 10 times :

for i in $(seq 1 10); do truncate -s 0  $(docker inspect -f='{{.LogPath}}' while-true); done

Show 20 last logs :

docker logs --tail=10 while-true
Sun Apr 11 17:29:35 UTC 2021
Sun Apr 11 17:29:35 UTC 2021
Sun Apr 11 17:29:36 UTC 2021
davidxxx
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  • Would that be OK even if the file is constantly being written into? – CIsForCookies Apr 11 '21 at 15:03
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    Yes You would not have any error. I updated with a basic example. – davidxxx Apr 11 '21 at 17:32
  • There's always a risk you'll time the truncate in the middle of the file being written, at which point logs will be broken like this commenter saw: https://stackoverflow.com/questions/42510002/how-to-clear-the-logs-properly-for-a-docker-container/42510314?noredirect=1#comment98713207_43570083 – BMitch Apr 11 '21 at 20:06
  • @ BMitch That is right but probably because of json log processing performed by Docker. That should work even in corrupted json : `cat $(docker inspect -f='{{.LogPath}}' foo-container)` – davidxxx Apr 12 '21 at 08:41
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The json-file logging driver does not rotate logs by default. In case you are looking on how to activate it instead of truncating them by hand, you can activate it by creating/updating /etc/docker/daemon.json:

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "15m",
    "max-file": "5",
  }
}

Note that you will need to restart docker (e.g. systemctl restart docker) and the config only affects containers created after modifying the daemon configuration (e.g. recreate the container of a specific service in docker-compose: docker-compose up --force-recreate --no-deps -d <service-name>).

For more information see:

Stuck
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