Docker Day 7: Persist Data with Volumes So It Survives Container Deletion

Problem: container removal deletes data

When a container is removed, any files written inside it disappear because the container's filesystem is temporary. Volumes provide a persistent data store on the host.

Storage options

Volumes – Docker‑managed persistent data. Command: docker volume create Bind mounts – Map a host directory into the container (useful for development). Command: -v /host:/container tmpfs – In‑memory storage for temporary sensitive data. Command:

--tmpfs /tmp

Volumes (recommended)

Creating and managing

# Create a volume
docker volume create mydata

# List all volumes
docker volume ls

# Inspect a volume
docker volume inspect mydata

# Remove unused volumes
docker volume prune

# Remove a specific volume
docker volume rm mydata

Using a volume in a container

# Method 1: specify at run time
docker run -d \
  -v mydata:/var/lib/postgresql/data \
  postgres:16-alpine

# Method 2: docker‑compose.yml
services:
  db:
    image: postgres:16-alpine
    volumes:
      - mydata:/var/lib/postgresql/data

volumes:
  mydata:

Anonymous vs. named volumes

# Anonymous volume (removed with container)
volumes:
  - /var/lib/postgresql/data

# Named volume (recommended)
volumes:
  - pg_data:/var/lib/postgresql/data

volumes:
  pg_data:

Bind mounts (development and debugging)

Map a host directory directly into the container so changes are reflected instantly.

# Format: -v host_path:container_path
docker run -it \
  -v $(pwd)/src:/app/src \
  -v $(pwd)/data:/app/data \
  python:3.12-slim bash

docker‑compose configuration

services:
  api:
    build: .
    volumes:
      - postgres_data:/var/lib/postgresql/data   # named volume for persistence
      - ./app:/app                               # bind mount for live code updates
      - ./config:/app/config:ro                 # read‑only mount

volumes:
  postgres_data:

Practical example: MySQL data persistence

Without a volume – data is lost when the container is removed.

docker run -d \
  --name mysql \
  -e MYSQL_ROOT_PASSWORD=secret \
  mysql:8.0

# Simulate data loss
docker rm -f mysql   # all data gone

With a named volume – data survives container removal.

services:
  mysql:
    image: mysql:8.0
    environment:
      MYSQL_ROOT_PASSWORD: secret
      MYSQL_DATABASE: app
    volumes:
      - mysql_data:/var/lib/mysql

volumes:
  mysql_data:

docker compose up -d
# create tables, insert data
docker compose down
docker compose up -d
# data is still present

Inspecting a volume

# Show the host path of a volume
docker volume inspect mysql_data
# Example output includes:
# {
#   "Mountpoint": "/var/lib/docker/volumes/mysql_data/_data",
#   "Name": "mysql_data",
#   ...
# }

Summary

Volumes : Docker‑managed persistent storage (recommended).

Bind mounts : Map host directories for real‑time development sync.

Removing a container does not delete data stored in a volume.

Use named volumes in production; avoid anonymous volumes or bind mounts for persistent data.