Docker Volumes Skill

Master Docker persistent storage including named volumes, bind mounts, tmpfs, and data backup/restore procedures.

Purpose

Implement reliable data persistence for containers with proper volume management, backup strategies, and permission handling.

Parameters

Storage Types

Volume Operations

Named Volumes

# Create volume
docker volume create app_data

# Use in container
docker run -d \
  -v app_data:/var/lib/postgresql/data \
  postgres:16-alpine

# Inspect volume
docker volume inspect app_data

# List volumes
docker volume ls

# Remove unused volumes
docker volume prune

Bind Mounts

# Development - mount source code
docker run -d \
  -v $(pwd)/src:/app/src \
  node:20-alpine

# Read-only config
docker run -d \
  -v /etc/app/config.yaml:/app/config.yaml:ro \
  myapp

tmpfs Mounts

# Sensitive data in memory
docker run -d \
  --tmpfs /app/secrets:rw,noexec,nosuid,size=64m \
  myapp

# Compose syntax
services:
  app:
    volumes:
      - type: tmpfs
        target: /app/tmp
        tmpfs:
          size: 100m

Docker Compose Volumes

services:
  database:
    image: postgres:16-alpine
    volumes:
      - db_data:/var/lib/postgresql/data
      - ./init.sql:/docker-entrypoint-initdb.d/init.sql:ro
    environment:
      POSTGRES_PASSWORD: ${DB_PASSWORD}

  app:
    image: myapp
    volumes:
      - uploads:/app/uploads
      - ./config:/app/config:ro

volumes:
  db_data:
    driver: local
  uploads:
    external: true  # Pre-created

Backup & Restore

Backup Volume

# Backup to tar file
docker run --rm \
  -v app_data:/source:ro \
  -v $(pwd)/backups:/backup \
  alpine tar cvf /backup/app_data_$(date +%Y%m%d).tar -C /source .

# Compressed backup
docker run --rm \
  -v app_data:/source:ro \
  -v $(pwd)/backups:/backup \
  alpine tar czvf /backup/app_data_$(date +%Y%m%d).tar.gz -C /source .

Restore Volume

# Restore from tar
docker run --rm \
  -v app_data:/dest \
  -v $(pwd)/backups:/backup:ro \
  alpine tar xvf /backup/app_data_20240101.tar -C /dest

Clone Volume

docker run --rm \
  -v source_volume:/from:ro \
  -v target_volume:/to \
  alpine cp -av /from/. /to/

Error Handling

Common Errors

volume in use

permission denied

no space left

path not found

Permission Fixes

# Check ownership
docker run --rm -v app_data:/data alpine ls -la /data

# Fix ownership (match container user)
docker run --rm -v app_data:/data alpine chown -R 1000:1000 /data

# SELinux (RHEL/CentOS)
docker run -v /host/path:/container/path:Z myimage

Troubleshooting

Debug Checklist

• Volume exists?

  docker volume ls

• Mount visible?

  docker exec <c> df -h

• Permissions correct?

  ls -la

  in container
• Data persisting? Stop/start container

Diagnostic Commands

# Check volume location
docker volume inspect app_data --format '{{.Mountpoint}}'

# Check container mounts
docker inspect <container> --format '{{json .Mounts}}'

# Check disk usage
docker system df -v

Usage

Skill("docker-volumes")

Related Skills

• docker-compose-setup
• docker-production