Container disk usage management in Docker - Step-by-Step Execution

Process Flow - Container disk usage management

Start: Check disk usage

↓

Run 'docker system df'

↓

Analyze output: Images, Containers, Volumes

↓

Decide cleanup action

↓

Remove unused images

↓

Remove stopped containers

↓

Remove unused volumes

↓

Run cleanup commands

↓

Verify disk usage reduced

↓

End

The flow shows checking disk usage, analyzing what takes space, choosing cleanup steps, running cleanup, and verifying results.

Execution Sample

Docker

docker system df
docker image prune -a
docker container prune
docker volume prune

This sequence checks disk usage, then removes unused images, stopped containers, and unused volumes.

Process Table

Step	Command Run	Action	Output Summary	Disk Usage Change
1	docker system df	Check disk usage	Shows space used by images, containers, volumes	No change
2	docker image prune -a	Remove all unused images	Deleted 3 images, freed 1.2GB	Disk usage decreased
3	docker container prune	Remove stopped containers	Deleted 2 containers, freed 500MB	Disk usage decreased
4	docker volume prune	Remove unused volumes	Deleted 1 volume, freed 300MB	Disk usage decreased
5	docker system df	Verify disk usage	Shows reduced space usage	Confirmed decrease
6	-	End	Cleanup complete	Final disk usage reduced

💡 Cleanup commands run and disk usage verified to be reduced.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Images (count)	10	7	7	7	7
Containers (stopped count)	5	5	3	3	3
Volumes (unused count)	4	4	4	3	3
Disk Usage (GB)	5.0	3.8	3.3	3.0	3.0

Key Moments - 3 Insights

Why does 'docker image prune -a' remove more images than just 'docker image prune'?

'docker image prune -a' removes all unused images including dangling and unreferenced ones, as shown in step 2 where 3 images are deleted, while 'docker image prune' removes only dangling images.

Why do we run 'docker system df' before and after cleanup?

Running 'docker system df' before shows current disk usage, and after cleanup confirms the disk space was freed, as seen in steps 1 and 5.

Does 'docker container prune' remove running containers?

No, it only removes stopped containers, which is why container count decreases from 5 to 3 after step 3.

Visual Quiz - 3 Questions

Look at the execution table, how many images were deleted after running 'docker image prune -a'?

A 5

B 2

C 3

D 0

💡 Hint

Check the 'Output Summary' column in step 2 of the execution table.

At which step does the disk usage first decrease?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Disk Usage Change' column in the execution table.

If no stopped containers existed, how would the 'docker container prune' step change?

A It would delete zero containers and free no space

B It would delete volumes instead

C It would delete containers anyway

D It would fail with an error

💡 Hint

Refer to the action and output in step 3 of the execution table.

Concept Snapshot

Container Disk Usage Management:
- Use 'docker system df' to check disk space used by images, containers, volumes.
- Clean unused data with 'docker image prune -a', 'docker container prune', 'docker volume prune'.
- Always verify disk usage after cleanup.
- Prune commands remove only unused or stopped resources.
- Regular cleanup prevents disk space issues.

Full Transcript

This visual execution shows how to manage disk usage in Docker containers. First, we check disk usage with 'docker system df' to see how much space images, containers, and volumes use. Then, we run cleanup commands: 'docker image prune -a' removes all unused images, 'docker container prune' removes stopped containers, and 'docker volume prune' removes unused volumes. After each cleanup, disk usage decreases as shown in the output summaries. Finally, we verify the disk usage again with 'docker system df' to confirm space was freed. Key points include understanding that prune commands only remove unused or stopped resources and that checking usage before and after cleanup helps confirm success.

Practice

1. What does the docker system df command show you?

easy

A. The disk space used by Docker images, containers, and volumes

B. The list of running Docker containers

C. The network settings of Docker containers

D. The logs of a specific Docker container

Solution

1. Step 1: Understand the purpose of docker system df and compare options

   This command summarizes disk usage by Docker objects like images, containers, and volumes. Options B (docker ps), C (network settings), and D (logs) describe unrelated commands.

2. Final Answer:

   The disk space used by Docker images, containers, and volumes -> Option A

3. Quick Check:

   Disk usage summary = A [OK]

Hint: Remember 'df' means disk free or usage summary [OK]

Common Mistakes:

• Confusing disk usage with container status
• Thinking it shows logs or network info
• Using it to list running containers

2. Which of the following commands correctly removes only all stopped containers to free disk space?

easy

A. docker container prune

B. docker image rm $(docker images -q)

C. docker system prune -a

D. docker container rm -f $(docker ps -a -q)

Solution

1. Step 1: Identify command to remove only stopped containers and check options

   docker container prune safely removes all stopped containers only. A forcibly removes all containers (running and stopped). B removes images. C removes stopped containers plus unused images and networks (more than containers).

2. Final Answer:

   docker container prune -> Option A

3. Quick Check:

   Remove stopped containers = D [OK]

Hint: Use prune commands to safely clean stopped containers [OK]

Common Mistakes:

• Using force remove on all containers including running ones
• Confusing image removal with container removal
• Using system prune when only containers should be removed

3. What will be the output of the command docker system df after running docker image prune -a?

medium

A. Error message about running containers

B. Disk usage report showing fewer images and freed space

C. List of all images including unused ones

D. No change in disk usage report

Solution

1. Step 1: Understand docker image prune -a effect on docker system df

   This removes all unused images, freeing space, so the report shows fewer images and more free space. No error, no list, and usage changes.

2. Final Answer:

   Disk usage report showing fewer images and freed space -> Option B

3. Quick Check:

   Image prune frees space = B [OK]

Hint: Prune unused images to reduce disk usage [OK]

Common Mistakes:

• Expecting an error when pruning images
• Thinking prune lists images instead of removing
• Assuming no change in disk usage after prune

4. You ran docker volume prune but still see high disk usage. What could be the reason?

medium

A. The prune command only cleans network settings

B. The prune command deleted running containers by mistake

C. Docker does not track volume disk usage

D. There are unused images and stopped containers still taking space

Solution

1. Step 1: Understand docker volume prune and reason for remaining usage

   It removes unused volumes only. Unused images and stopped containers remain, causing high usage. No deletion of running containers or network cleaning.

2. Final Answer:

   There are unused images and stopped containers still taking space -> Option D

3. Quick Check:

   Volume prune only removes volumes = A [OK]

Hint: Prune volumes only frees volume space, check images and containers too [OK]

Common Mistakes:

• Assuming volume prune deletes containers
• Thinking prune affects network settings
• Believing Docker ignores volume disk usage

5. You want to free maximum disk space but keep your running containers and their data intact. Which command should you use?

hard

A. docker container rm -f $(docker ps -q)

B. docker system prune --volumes

C. docker system prune -a --volumes

D. docker image rm $(docker images -q)

Solution

1. Step 1: Analyze options for maximum safe cleanup

   docker system prune -a --volumes removes all stopped containers, all unused images (not just dangling), unused networks, and unused volumes--maximizing space--while keeping running containers and their data intact. C frees less (no -a). A removes running containers. D only images, unsafely and incompletely.

2. Final Answer:

   docker system prune -a --volumes -> Option C

3. Quick Check:

   Max prune safely, keep running containers = B [OK]

Hint: Use system prune with volumes to clean safely without stopping containers [OK]

Common Mistakes:

• Using prune without -a flag, freeing less space
• Force removing running containers causing downtime
• Removing images only without cleaning volumes or containers