Why Should We Copy Docker Volumes?

This is the most common scenario. You may want to clone the running db_data volume to a test volume called db_data_test.

🤔 WHY: Why Use a Temporary Container?

Docker volumes are stored somewhere in the host's file system (e.g., /var/lib/docker/volumes/...), but the Docker daemon manages this path. It's not recommended to directly access this path and use the cp command, as it can lead to permission issues or data consistency problems.

Instead, we execute a temporary container that mounts two volumes simultaneously.

1. The container mounts the source volume (source_volume) to the /from path.

2. The container mounts the target volume (new_volume) to the /to path.

3. The container immediately executes a simple Linux command (cp or tar) to copy all data from /from to /to.

4. Once the command finishes, the container is automatically discarded (--rm option).

This container is used solely as a 'tool' for data copying.

🚀 HOW: Command Example

Here is an example of copying data from the source_data volume to the target_data volume.

1. Create Test Volume (Optional)

docker volume create source_data
docker volume create target_data
# (Assuming that there is data in source_data.)

1. Copy Using a Temporary Container

docker run --rm \
       -v source_data:/from \
       -v target_data:/to \
       alpine \
       sh -c "cp -a /from/. /to/"

💡 Command Explanation

• docker run --rm: The --rm flag means to immediately delete the container once the task is complete. This is essential for temporary tasks.

• -v source_data:/from: Mounts the source_data volume to the /from directory within the container.

• -v target_data:/to: Mounts the target_data volume to the /to directory within the container.

• alpine: Uses the alpine Linux image, which is very small yet includes basic utilities like cp and sh.

• sh -c "...": The command that will run when the alpine container starts.

• cp -a /from/. /to/:

  • -a (archive): This flag is crucial. Instead of a simple copy (cp -r), it preserves all attributes such as ownership, permissions, and timestamps during the copy. This is very important when dealing with sensitive data like database files.

  • /from/.: Refers to all contents inside the /from directory (including hidden files) rather than the directory itself.

2. Copy Volume Data to Host File System (Backup)

This is used when you want to back up the volume data to a specific directory on your local machine or server as a .tar file.

🤔 WHY: How Does It Work?

The principle is the same as in point 1; only the target has changed from 'Docker volume' to 'host directory'.

Docker supports a bind mount feature that mounts a specific directory from the host into the container.

1. The container mounts the source volume (source_volume) to the /data path.

2. The container mounts a specific directory on the host ($(pwd)/backup) to the /backup path.

3. The container copies (or compresses) the contents of /data to the /backup directory.

4. This action happens within the container, but since /backup is linked to the actual host directory, the results remain on the host.

🚀 HOW: Command Example

Copies the data from the source_data volume to the backup directory in the current location ($(pwd)).

# Create backup directory on the host
mkdir -p $(pwd)/backup

docker run --rm \
       -v source_data:/data:ro \
       -v $(pwd)/backup:/backup \
       alpine \
       cp -a /data/. /backup/

💡 Command Explanation

• -v source_data:/data:ro: The :ro (Read-Only) flag has been added. Since backup only needs to read the data, it is advisable to mount it as read-only to prevent accidental modification of the source volume.

• -v $(pwd)/backup:/backup: Unlike source_data (volume name), the / beginning or $(pwd) (current path) indicates this is an absolute/relative path that refers to the host's directory.

3. Key Copy Tips

docker run --rm \
       -v source_data:/from \
       -v target_data:/to \
       alpine \
       sh -c "cd /from && tar -cf - . | (cd /to && tar -xf -)"

docker run --rm \
       -v source_data:/data:ro \
       -v $(pwd):/backup \
       alpine \
       tar -czf /backup/backup.tar.gz -C /data .

# Restore data from the $(pwd)/backup directory to new_data volume
docker run --rm \
       -v new_data:/data \
       -v $(pwd)/backup:/backup:ro \
       alpine \
       cp -a /backup/. /data/