Docker Data Persistence

1. The Core Concept: Ephemeral Containers

By default, containers are ephemeral. This means:

• Files created inside a container are stored in a writable layer.
• When the container is deleted (docker rm), that writable layer — and all your data — is destroyed.

Solution: We use Storage Mounts to persist data beyond the life of the container.

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2. Docker Volumes (The Production Standard)

Volumes are the preferred mechanism for persisting data generated by and used by Docker containers.

• Managed by: Docker
  (Location: /var/lib/docker/volumes/ on Linux)
• Best for: Databases, application data, and production environments.
• Behavior: Even if the container is removed, the Volume (and its data) remains until explicitly deleted.

Create a New Volume

docker volume create <volume_name>

List All Volumes

docker volume ls

Inspect a Volume

docker volume inspect <volume_name>

Look for the “Mountpoint” entry.

Usage Example

Mounting a volume named arch-data to /data inside the container:

docker run -v arch-data:/data alpine ...

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3. Bind Mounts (The Developer Standard)

Bind mounts map a specific file or directory on the host machine to a file or directory inside the container.

• Managed by: You (the user)
• Best for: Local development, live code reloading, editing config files
• Behavior: Changes on the host are instantly visible in the container — and vice versa.

Usage Example

Mounting the current directory ($(pwd)) on the host to the Nginx HTML folder:

docker run -v $(pwd):/usr/share/nginx/html nginx

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4. Volumes vs Bind Mounts (Comparison)

Feature	Volumes	Bind Mounts
Host Location	Managed by Docker (/var/lib/...)	Any path you specify
Portability	High (Works on any OS)	Low (Host dependent)
Use Case	Database storage, safety	Live coding, config injection

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5. Critical CLI Flags & Concepts

The --rm Flag

Automatically removes the container when it exits.

docker run --rm alpine echo "I run, then I disappear"

Why use it?
Prevents your disk from filling up with stopped “zombie” containers during testing.

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The sh -c Command

Used to pass a command string to a shell inside the container.

Scenario

You want to run:

echo "hello" > file.txt

Wrong

docker run alpine echo "hello" > file.txt

Result:
The > is interpreted by your host shell.
The file is created on your local machine.

Right

docker run alpine sh -c "echo 'hello' > file.txt"

Result:
The entire command runs inside the container, and the file is created inside the container filesystem.