Docker Volumes Skill

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

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