Lesson 2: The Backup Artist (Volume Operations)

Data is only as safe as your last backup. In Docker, volumes need the same care as any database — regular backups, monitoring, and cleanup.

Backing Up a Volume

Docker doesn't have a built-in docker volume backup command. Instead, you use a helper container:

docker run --rm \
  -v app-data:/data \
  -v $(pwd):/backup \
  alpine tar czf /backup/data-backup.tar.gz /data

What this does:

1. Starts a temporary Alpine container.
2. Mounts the app-data volume at /data.
3. Mounts your current directory at /backup.
4. Creates a compressed archive of the volume contents.
5. --rm auto-removes the helper container when done.

Restoring a Volume

# Create a fresh volume
docker volume create app-data-restored

# Restore from backup
docker run --rm \
  -v app-data-restored:/data \
  -v $(pwd):/backup \
  alpine tar xzf /backup/data-backup.tar.gz -C /

Exploring Volume Contents

Sometimes you just need to peek inside:

# List files
docker run --rm -v app-data:/data alpine ls -la /data

# Read a specific file
docker run --rm -v app-data:/data alpine cat /data/config.json

# Get a shell to browse
docker run --rm -it -v app-data:/data alpine sh

Docker Disk Usage

Docker can eat up disk space fast. Monitor it:

docker system df

TYPE            TOTAL    ACTIVE   SIZE      RECLAIMABLE
Images          15       3        4.2GB     3.1GB (73%)
Containers      5        2        150MB     120MB (80%)
Local Volumes   8        2        2.1GB     1.8GB (85%)
Build Cache     -        -        500MB     500MB

The Nuclear Option: Full Cleanup

# Remove stopped containers, unused images, networks, and volumes
docker system prune -a --volumes

⚠️ Warning: This removes everything not currently in use!

Development Workflow with Bind Mounts

For local development, bind mounts give you live reloading:

docker run -d \
  --name dev-server \
  -v $(pwd)/src:/app/src \
  -v $(pwd)/package.json:/app/package.json \
  -p 3000:3000 \
  node:18-alpine \
  sh -c "cd /app && npm install && npm run dev"

Edit files on your laptop → changes appear instantly in the container.

Mission Objective

Master volume operations:

1. Backup: Run the backup command to archive your volume to data-backup.tar.gz.
2. Explore: List files inside the volume with a helper container.
3. Monitor: Run docker system df to check disk usage.

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