Volumes

Docker Volumes Cheat Sheet

Overview

Volumes are used to persist data in Docker containers. Unlike bind mounts, volumes are managed by Docker, and they are a preferred way to store data across container restarts or upgrades.

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Key Concepts

• Named Volumes: Managed by Docker and can be shared between multiple containers.
• Anonymous Volumes: Automatically created by Docker, useful for temporary data.
• Bind Mounts: Links a specific directory on the host machine to a directory inside the container.

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Commands for Managing Volumes

Command	Description
docker volume create <name>	Create a named volume.
docker volume ls	List all volumes.
docker volume inspect <name>	Display details about a volume.
docker volume rm <name>	Remove a specific volume.
docker volume prune	Remove all unused volumes.

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Using Volumes in Containers

Attach a Volume to a Container

docker run -d --name my_container -v my_volume:/app/data my_image

• my_volume: The name of the volume.
• /app/data: The directory inside the container where the volume is mounted.

Using Bind Mounts

docker run -d --name my_container -v /path/to/host:/app/data my_image

• /path/to/host: Path on the host machine.
• /app/data: Directory inside the container.

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Managing Volumes in docker-compose.yml

Define Named Volumes

version: '3.8'

services:
  app:
    image: my_app
    volumes:
      - my_named_volume:/app/data

volumes:
  my_named_volume:

Example with Multiple Services

version: '3.8'

services:
  app:
    image: node:16
    volumes:
      - app_data:/app
    depends_on:
      - db

  db:
    image: postgres:13
    volumes:
      - db_data:/var/lib/postgresql/data

volumes:
  app_data:
  db_data:

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Examples of Volume Usage

Persist Database Data

Run a PostgreSQL container with a persistent data volume:

docker run -d \
  --name postgres_container \
  -e POSTGRES_USER=admin \
  -e POSTGRES_PASSWORD=secret \
  -v postgres_data:/var/lib/postgresql/data \
  postgres:latest

• postgres_data: Named volume to persist database data.

Share Data Between Containers

1. Start the first container:

   docker run -d --name container1 -v shared_volume:/data alpine

2. Start the second container using the same volume:

   docker run -d --name container2 -v shared_volume:/data alpine

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Inspecting and Cleaning Volumes

Inspect a Volume

docker volume inspect my_volume

Prune Unused Volumes

Remove all volumes not referenced by any container:

docker volume prune

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Best Practices

1. Use Named Volumes: For data that needs to persist and be managed by Docker.
2. Avoid Bind Mounts for Sensitive Data: Bind mounts directly link host directories and can expose the host file system.
3. Keep Volumes Organized: Use meaningful names to identify volumes easily.
4. Clean Up Unused Volumes: Regularly prune unused volumes to free up disk space.

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Advanced Example: Using Volumes with Backups

Create a Volume Backup

1. Run a container to create a backup:

   docker run --rm \
     -v my_volume:/volume_data \
     -v /path/to/backup:/backup \
     alpine tar czf /backup/my_volume_backup.tar.gz /volume_data

Restore a Volume Backup

1. Restore from the backup file:

   docker run --rm \
     -v my_volume:/volume_data \
     -v /path/to/backup:/backup \
     alpine tar xzf /backup/my_volume_backup.tar.gz -C /volume_data

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Volume-Related Issues and Debugging

1. Check Mounted Volumes:

   docker inspect <container_id>

2. Verify Data is Persisted: Stop the container and restart it. Check if the data remains in the volume.
3. Remove Stale Volumes: Use docker volume prune to clean up unused volumes.