Docker fills a VPS disk in three ways: the images and layers left behind after updates, the volumes nobody uses, and the container logs, which by default grow without end. The typical error is “No space left on device”, and the symptom is a database that stops or a container that will not start. First you measure; then you clear only what you are sure you can lose.
First: where is the space
| 1 |
See the whole disk: df -h. Find the line for the main partition and the “Use%” column.
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| 2 |
See what belongs to Docker: docker system dfIt shows images, containers, volumes and the build cache, and how much of the space is reclaimable.
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| 3 |
See the largest directories on the system, if Docker does not explain it all: sudo du -xh --max-depth=1 /var | sort -hDocker’s data normally lives in /var/lib/docker.
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Cleaning up, from safest to riskiest
| What takes space |
Command |
Risk |
| Untagged or unused images |
docker image prune |
Low: it pulls again whatever you need. |
| Stopped containers |
docker container prune |
Low, but check what is there first with docker ps -a. |
| Build cache |
docker builder prune |
Low: later builds are slower. |
| Every image no container uses |
docker image prune -a |
Medium: you will have to pull them again. |
| Volumes with no container |
docker volume ls and only then docker volume rm name |
High. A volume may hold your database. Delete one at a time, knowing what it is. |
The logs that grow without end
Each container writes whatever the application prints to a log file on the server, and by default that file has no limit. A chatty application, or one failing in a loop, can fill the disk with that alone. To see it: sudo du -sh /var/lib/docker/containers. The fix is to cap the size and the number of files, in Compose:services:
app:
logging:
driver: json-file
options:
max-size: "10m"
max-file: "3"The values are examples; pick your own. The rule applies to containers created after the change: recreate them with docker compose up -d --force-recreate. To free the space of a gigantic log right now, as an emergency: sudo truncate -s 0 $(docker inspect --format="{{.LogPath}}" container-name).
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Beware of docker system prune -a --volumes. It is the command that appears in every guide and the one that most often deletes a database. Use it only on a server where you know exactly what is in each volume, and with a backup made. When in doubt, the safer commands in the table above are enough.
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