Why data persistence matters in Docker - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a Docker container that keeps data after it stops.

Docker

docker run -d --name mycontainer -v [1]:/data busybox

Drag options to blanks, or click blank then click option'

A/host/data

B/container/data

C/tmp

D/var/log

Attempts:

3 left

💡 Hint

Common Mistakes

Using a container path instead of a host path for volume mapping.

The -v option maps a host folder to the container to keep data persistent.

2fill in blank

medium

Complete the command to create a named Docker volume called 'myvol'.

Docker

docker volume [1] myvol

Drag options to blanks, or click blank then click option'

Acreate

Blist

Cinspect

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' or 'list' instead of 'create'.

The create command makes a new named volume in Docker.

3fill in blank

hard

Fix the error in the Docker run command to properly mount a volume named 'myvol' to '/app/data'.

Docker

docker run -d --name appcontainer -v [1]:/app/data busybox

Drag options to blanks, or click blank then click option'

Avolumes/myvol

Bvolume:myvol

Cmyvol

D/myvol

Attempts:

3 left

💡 Hint

Common Mistakes

Adding slashes or prefixes to the volume name.

To mount a named volume, just use its name without slashes or prefixes.

4fill in blank

hard

Fill both blanks to create a Docker volume and run a container using it.

Docker

docker volume [1] mydata && docker run -d -v [2]:/data alpine

Drag options to blanks, or click blank then click option'

Acreate

Bmydata

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' instead of 'create' or wrong volume name.

First, create the volume with create. Then use the volume name mydata to mount it.

5fill in blank

hard

Fill all three blanks to define a Docker Compose service with a volume for data persistence.

Docker

services:
  db:
    image: postgres
    volumes:
      - [1]:[2]
volumes:
  [3]:

Drag options to blanks, or click blank then click option'

Adbdata

B/var/lib/postgresql/data

Dmyvolume

Attempts:

3 left

💡 Hint

Common Mistakes

Using different volume names or wrong container paths.

The volume name dbdata is used in both service and volumes sections. The container path is /var/lib/postgresql/data.

Practice

1. Why is data persistence important when using Docker containers?

easy

A. Because containers automatically save data to the host system

B. Because data persistence makes containers run faster

C. Because Docker containers never delete data inside them

D. Because containers are temporary and data is lost when they stop

Solution

1. Step 1: Understand container lifecycle

   Docker containers are designed to be temporary and can be stopped or removed easily.

2. Step 2: Recognize data loss risk

   When a container stops or is deleted, any data stored inside it is lost unless persisted externally.

3. Final Answer:

   Because containers are temporary and data is lost when they stop -> Option D

4. Quick Check:

   Temporary containers lose data without persistence [OK]

Hint: Remember: containers reset, so save data outside [OK]

Common Mistakes:

• Thinking containers save data automatically
• Assuming data inside container is permanent
• Confusing persistence with performance

2. Which Docker command option is used to create a persistent data volume?

easy

A. docker run -v /host/path:/container/path

B. docker run --rm

C. docker run --network=host

D. docker run --memory=512m

Solution

1. Step 1: Identify volume option

   The -v or --volume flag maps a host directory to a container directory for persistence.

2. Step 2: Check other options

   --rm removes container after exit, --network=host sets network mode, --memory limits memory; none create persistent storage.

3. Final Answer:

   docker run -v /host/path:/container/path -> Option A

4. Quick Check:

   Volume flag = -v [OK]

Hint: Use -v to link host folder for persistence [OK]

Common Mistakes:

• Using --rm which deletes container and data
• Confusing network or memory flags with storage
• Not specifying host path in volume mapping

3. What will happen if you run this command twice without a volume?

docker run --rm alpine sh -c "echo hello > /tmp/file.txt"

medium

A. The file will be lost after the container stops

B. The file will be saved on the host automatically

C. The container will fail to start the second time

D. The file will persist and contain 'hello' after both runs

Solution

1. Step 1: Analyze container data storage

   Without a volume, data inside the container is stored in the container's writable layer, which is deleted when container is removed.

2. Step 2: Consider container restart

   Running the command twice creates new containers or overwrites existing ones, losing previous data inside.

3. Final Answer:

   The file will be lost after the container stops -> Option A

4. Quick Check:

   No volume means no data persistence [OK]

Hint: No volume means data lost on container stop [OK]

Common Mistakes:

• Assuming data inside container persists automatically
• Thinking container names prevent data loss
• Believing host saves container files without volume

4. You want to persist data but your Docker container loses files after restart. What is the most likely mistake?

medium

A. You used --rm flag when running the container

B. All of the above

C. You mapped a volume but to a wrong host directory

D. You forgot to map a volume with -v option

Solution

1. Step 1: Check volume mapping

   Not using -v means no data persistence outside container.

2. Step 2: Consider container removal flags and paths

   --rm deletes container and data on exit; wrong host path means data saved elsewhere.

3. Final Answer:

   All of the above -> Option B

4. Quick Check:

   Missing or wrong volume or --rm causes data loss [OK]

Hint: Check volume and flags carefully to keep data [OK]

Common Mistakes:

• Ignoring --rm deletes container and data
• Mapping volume to wrong or non-existent host path
• Assuming volume mapping alone guarantees persistence

5. You have a Docker container that writes logs to /app/logs. How do you ensure logs persist on the host even if the container is removed?

hard

A. Copy logs manually after container stops

B. Run the container with docker run --rm

C. Run the container with docker run -v /host/logs:/app/logs

D. Use docker commit to save container state

Solution

1. Step 1: Map container logs directory to host

   Using -v /host/logs:/app/logs links container logs to host folder for persistence.

2. Step 2: Avoid temporary container flags

   --rm deletes container and logs; copying manually is error-prone; docker commit saves image, not files.

3. Final Answer:

   Run the container with docker run -v /host/logs:/app/logs -> Option C

4. Quick Check:

   Volume mapping keeps logs safe [OK]

Hint: Map logs folder with -v for safe persistence [OK]

Common Mistakes:

• Using --rm deletes container and logs
• Relying on manual copying which is error-prone
• Misunderstanding docker commit saves files