8.1 Introduction to Virtual Disks
Mounting volumes is one of the coolest features of Docker. It lets containers easily work with data from the host machine. This makes data management flexible and convenient: you can save data for long-term use, share it between containers, and easily back up or restore data if something goes wrong.
Volumes in Docker are a way to store data so that it doesn't depend on containers. Even if a container is deleted, the info stored in volumes remains safe. You can attach (mount) volumes to containers, allowing you to store data separately and reuse it no matter how many times you create or remove containers.
Key Volume Types
- Anonymous Volumes
- Automatically created by Docker if no volume is explicitly specified for a container.
- Used for temporary data storage.
- Named Volumes
- Created and managed by Docker.
- Can be attached to multiple containers and persist even after they’re deleted or stopped.
- Bind Mounts
- Link a directory on the host machine to a directory in the container.
- Perfect for accessing host system data and handy during development.
Basic Syntax of the docker run Command for Mounting Volumes
-v <host_path>:<container_path>
Full Syntax:
docker run -v <host_path>:<container_path> [OPTIONS] IMAGE [COMMAND] [ARG...]
Main Parameters for Mounting Volumes
1. Parameter -v or --volume
Used for creating a named volume or a bind mount.
Example:
docker run -d -v /host/data:/container/data nginx
2. Parameter --mount
Provides a more flexible and detailed way to mount volumes, supporting additional parameters.
Example:
docker run -d --mount type=bind,source=/host/data,target=/container/data nginx
8.2 Creating and Using Volumes
1. Named Volumes
Named volumes are created and managed by Docker. They're used for long-term storage of data that should be retained between container restarts or removals.
Creating a named volume:
docker volume create my_volume
Running a container with a mounted volume:
In this example, the volume my_volume is mounted to the directory /data inside the container my_container. All data written to /data will be preserved in the volume and remain accessible even after the container is removed.
docker run -d -v my_volume:/data --name my_container nginx
2. Anonymous Volumes
Anonymous volumes are created by Docker automatically and are tied to a specific container. They are useful for temporary data that doesn't need to be retained after the container is removed.
Running a container with an anonymous volume:
In this example, Docker will automatically create an anonymous volume and mount it to the directory /data inside the container.
docker run -d -v /data --name my_container nginx
3. Bind Mounts
Bind mounts allow you to mount directories on the host system into containers. This is handy for sharing data between containers and the host system. It's also often used during development when the source code is stored on the host system.
Running a container with a bind mount:
In this example, the directory /host/data on the host machine is mounted to the directory /container/data inside the container my_container.
docker run -d -v /host/data:/container/data --name my_container nginx
8.3 Examples of Using Volumes
1. Database Data Persistence
Using volumes for databases helps keep the data intact even during container restarts or updates. This is especially crucial for reliable operation in production environments.
Example:
In this example, PostgreSQL data is stored in the db_data volume, ensuring its preservation during container restarts or deletion.
docker volume create db_data
docker run -d -v db_data:/var/lib/postgresql/data --name postgres_container postgres
2. Sharing Data Between Containers
Sometimes it's necessary to share data between multiple containers. Volumes make this easy to achieve.
Example
Let's create a volume and run two containers that will use this volume. Both containers will have access to the data in the shared_data volume, allowing them to share information.
docker volume create shared_data
docker run -d -v shared_data:/data --name container1 nginx
docker run -d -v shared_data:/data --name container2 nginx
3. Development and Testing
When developing applications, it's handy to use bind mounts to share source code between the container and the host machine. This allows developers to edit code on the host, and the container can immediately use the updates.
Example
In this example, the /path/to/source directory on the host machine is mounted into the /app directory inside the dev_container. This way, the container can work with the actual source code directly from the host system.
docker run -d -v /path/to/source:/app --name dev_container node
8.4 Managing Volumes
Docker provides convenient commands for managing volumes, making it easy to create, delete, and view information about volumes.
1. Viewing all volumes
This command lists all available volumes on the host machine.
docker volume ls
2. Viewing volume information
With this command, you can get detailed info about the volume my_volume: its location on the host file system, as well as information about containers using this volume.
docker volume inspect my_volume
3. Removing a volume
This command removes the volume my_volume. Before removing, it's important to ensure that the volume isn't being used by any container, otherwise, the command won't execute.
docker volume rm my_volume
8.5 Backing up volumes
To keep your data safe, it's super important to be able to back up your volumes and restore 'em when you need to.
1. Backing up a volume
In this example, the content of the volume my_volume gets archived into a file my_volume_backup.tar.gz, which is saved in the /backup directory on the host machine.
docker run --rm -v my_volume:/volume -v /backup:/backup busybox tar czf /backup/my_volume_backup.tar.gz /volume
2. Restoring a volume
This example restores the content of the volume my_volume from the backup stored in the file my_volume_backup.tar.gz.
docker run --rm -v my_volume:/volume -v /backup:/backup busybox tar xzf /backup/my_volume_backup.tar.gz -C /volume
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