Container disk usage management in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Check disk space used by Docker images

What is the output of the command docker system df -v regarding image sizes?

Docker

docker system df -v

A Shows Docker daemon logs related to disk usage

B Only lists running containers with their disk usage

C Displays network usage statistics for Docker containers

D Shows detailed disk usage including images, containers, volumes, and build cache with sizes

💡 Hint

Think about which command gives a detailed summary of disk usage for all Docker objects.

The docker system df -v command provides a verbose summary of disk usage by images, containers, volumes, and build cache. It helps identify which objects consume the most space.

❓ Troubleshoot

intermediate

Identify cause of disk space not freeing after container removal

You removed a Docker container but disk space did not increase. What is the most likely reason?

A The container was not actually stopped before removal

B Docker daemon needs to be restarted to free space

C The container's volumes are still present and consuming space

D Docker images are automatically deleted with container removal

💡 Hint

Think about what Docker volumes do when containers are deleted.

Removing a container does not delete its associated volumes by default. Volumes store persistent data and remain on disk, consuming space until explicitly removed.

❓ Configuration

advanced

Configure Docker to limit container log size

Which Docker daemon configuration snippet correctly limits container log files to 10MB with 3 rotated files?

A

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "100m",
    "max-file": "1"
  }
}

B

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}

C

{
  "log-driver": "syslog",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}

D

{
  "log-driver": "journald",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}

💡 Hint

The default log driver that supports size and file rotation is json-file.

The json-file log driver supports max-size and max-file options to limit log file size and number of rotated files. Other drivers like syslog or journald do not support these options.

🔀 Workflow

advanced

Steps to safely clean up unused Docker resources

Which option lists the correct sequence of commands to safely clean up unused containers, images, volumes, and build cache?

A docker container prune → docker image prune → docker volume prune → docker builder prune

B docker volume prune → docker container prune → docker builder prune → docker image prune

C docker system prune -a → docker container prune → docker volume prune → docker builder prune

D docker builder prune → docker image prune → docker container prune → docker volume prune

💡 Hint

Start by removing stopped containers, then images, volumes, and finally build cache.

The recommended order is to prune stopped containers first, then unused images, followed by unused volumes, and finally the build cache. This order avoids dependency conflicts and safely frees disk space.

✅ Best Practice

expert

Prevent excessive disk usage by containers in production

Which practice best prevents excessive disk usage by Docker containers in a production environment?

A Use read-only root filesystem for containers and mount writable volumes only where needed

B Disable Docker logging to prevent log files from growing

C Run containers with unlimited disk quota to avoid failures

D Avoid using volumes and store all data inside containers

💡 Hint

Think about controlling where data is written to limit disk growth.

Using a read-only root filesystem limits container writes to only designated volumes. This prevents unexpected disk usage growth inside containers. Unlimited disk quota or disabling logs can cause issues. Avoiding volumes leads to data loss and harder management.

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