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

Parameter	Type	Required	Default	Description
volume_name	string	No	-	Name for the volume
mount_type	enum	No	volume	volume/bind/tmpfs
backup	boolean	No	false	Include backup commands

Storage Types

Type	Persistence	Use Case	Managed By
Named Volume	Yes	Production data	Docker
Bind Mount	Yes	Development, configs	Host
tmpfs	No (memory)	Secrets, temp files	Docker
Anonymous	Container lifetime	Scratch space	Docker

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

Error	Cause	Solution
volume in use	Container running	Stop container first
permission denied	UID mismatch	Fix ownership
no space left	Disk full	Clean up or expand
path not found	Bind mount missing	Create directory

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