How to view and clean up Docker container logs (tested and effective)

1. Problem

The docker container logs caused the host disk space to be full. docker logs -f container_name has a lot of crackling logs, which takes up a lot of space. Unused logs can be cleaned up.

2. Solution

2.1 Find Docker container logs

On Linux, container logs are generally stored in /var/lib/docker/containers/container_id/. Files ending with json.log (business logs) are very large. The script docker_log_size.sh to check the size of each log file is as follows:

#!/bin/sh

echo "======== docker containers logs file size ========" 

logs=$(find /var/lib/docker/containers/ -name *-json.log) 

for log in $logs 
    do 
       ls -lh $log  
    done 

# chmod +x docker_log_size.sh

# ./docker_log_size.sh

2.2 Cleaning up Docker container logs (treating the symptoms)

If the Docker container is running, after deleting the log using rm -rf, you will find that the disk space is not released through df -h. The reason is that in Linux or Unix systems, deleting files through rm -rf or the file manager will unlink them from the directory structure of the file system. If the file is open (in use by a process), the process will still be able to read the file and the disk space will continue to be occupied. The correct approach is cat /dev/null > *-json.log. Of course, you can also delete it with rm -rf and restart docker. Next, provide a log cleaning script clean_docker_log.sh, the content is as follows:

#!/bin/sh 

echo "======== start clean docker containers logs ========" 

logs=$(find /var/lib/docker/containers/ -name *-json.log) 

for log in $logs 
    do 
        echo "clean logs : $log" 
        cat /dev/null > $log 
    done 

echo "======== end clean docker containers logs ========"

# chmod +x clean_docker_log.sh

# ./clean_docker_log.sh

However, after such cleanup, container logs will come back like weeds over time.

2.3 Setting the Docker container log size (root cause)

• Set the upper limit of the log size for a container service

With the above method, the log file will increase in size sooner or later. To fundamentally solve the problem, you need to limit the upper limit of the log size of the container service. This is achieved by configuring the max-size option of the container docker-compose

nginx: 
 image: nginx:1.12.1 
 restart: always 
 logging: 
  driver: "json-file" 
  options: 
   max-size: "5g" 

After restarting the nginx container, the size of its log file is limited to 5GB, so you don’t have to worry about it anymore.

• Global Settings

Create a new /etc/docker/daemon.json. If it already exists, you don’t need to create a new one. Add the log-dirver and log-opts parameters, as shown below:

# vim /etc/docker/daemon.json

{
 "registry-mirrors": ["http://f613ce8f.m.daocloud.io"],
 "log-driver":"json-file",
 "log-opts": {"max-size":"500m", "max-file":"3"}
}

max-size=500m means that the upper limit of a container log size is 500M.

max-file=3 means that a container has three logs, namely id+.json, id+1.json, and id+2.json.

// Restart the Docker daemon# systemctl daemon-reload

# systemctl restart docker

Note: The set log size is only valid for newly created containers.

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