In Docker, a storage driver is responsible for managing how container data is stored on the host filesystem. Docker uses storage drivers to implement the container filesystem, allowing containers to have isolated, persistent storage while still being lightweight and portable.

Each Docker container has its own filesystem that’s separate from the host machine and other containers. The storage driver determines how this filesystem is created, managed, and interacts with the underlying operating system.

Key Concepts of Docker Storage Drivers

1. Union Filesystem: Docker storage drivers are based on the concept of a union filesystem. This allows different layers of a container image to be stacked on top of one another. Each layer is read-only, except the top layer, which is writable and used for any changes made by the container.

2. Layering: Images are made up of several layers (e.g., a base OS, additional software, and configurations), and each layer is stored as a separate filesystem object. These layers are shared among containers, which helps reduce storage usage by making containers smaller and faster to deploy.

3. Copy-on-Write (CoW): Most Docker storage drivers use a Copy-on-Write mechanism, where when a container makes a change to its filesystem, it only copies the part of the filesystem that’s being modified, leaving the other layers unchanged. This makes it more efficient in terms of space and speed.

Types of Docker Storage Drivers

Docker supports several storage drivers, each with different performance and features. The choice of storage driver depends on the underlying operating system, the use case, and performance requirements. The most common storage drivers are:

1. OverlayFS (overlay2)

• Recommended for most systems: overlay2 is the default storage driver for Docker on most Linux distributions and is often the best choice in terms of performance and efficiency.
• Key features:
  • Uses a union filesystem.
  • Works by combining multiple directories (layers) in a way that looks like a single directory to the container.
  • It’s lightweight and efficient since it only uses copy-on-write (CoW) for changes to the filesystem.
  • Suitable for modern Linux kernels (>= 4.0).
• Usage:
  • overlay2 is preferred on systems with kernel 4.0 or later, as it provides better performance and scalability than other drivers.

2. AUFS (Advanced Multi-Layered Unification Filesystem)

• Older, deprecated: aufs was one of the original storage drivers used by Docker but is now considered deprecated.
• Key features:
  • Similar to OverlayFS in that it uses a union filesystem to combine layers.
  • Provides support for several different layers, and it uses copy-on-write to keep the system efficient.
  • Typically has better support for advanced features compared to other drivers.
• Usage:
  • AUFS is not included in the default installation on many newer distributions, and Docker recommends overlay2 for most users.

3. Btrfs

• Linux-specific filesystem: btrfs is a modern filesystem with advanced features like snapshots, compression, and checksumming.
• Key features:
  • Supports advanced features like snapshots, which allow you to create and manage container snapshots.
  • Can provide higher performance in certain use cases but is not as widely used as overlay2.
  • Often requires more maintenance and has less support in production environments compared to overlay2.
• Usage:
  • Suitable for advanced users or cases where specific features (like snapshots) are needed.
  • Often used on systems where the Btrfs filesystem is already in use.

4. Device Mapper

• Block-level storage: The devicemapper storage driver operates at the block level, managing data in a pool of block devices.
• Key features:
  • Offers advanced features like thin provisioning and block-level snapshots.
  • Not as performant as other drivers like overlay2 or aufs.
  • Requires careful configuration of storage space, as it can be prone to running out of space if not managed properly.
• Usage:
  • Often used for environments that need block-level storage and where overlay2 is not available.

5. ZFS

• High-performance, advanced filesystem: zfs is a high-performance filesystem with advanced features like snapshots, compression, and deduplication.
• Key features:
  • Supports advanced storage features such as data integrity checking, compression, and snapshotting.
  • Typically used in high-performance environments or when advanced storage features are required.
  • Less common than other drivers, but useful for specialized use cases.
• Usage:
  • Primarily used on systems where ZFS is already in use, and where the advanced features it offers are necessary.

How to Check the Storage Driver in Use

To check which storage driver Docker is using on your system, you can run the following command:

docker info | grep "Storage Driver"

This will display the storage driver currently in use by Docker.

How to Set a Storage Driver

When you install Docker, it automatically selects a default storage driver based on the available filesystem and kernel features. However, you can specify a different storage driver by modifying Docker’s configuration.

1. Edit Docker’s daemon configuration:

   Open /etc/docker/daemon.json and specify the desired storage driver. For example, to use overlay2:

   {  "storage-driver": "overlay2"}
   

2. Restart Docker to apply the change:

   sudo systemctl restart docker
   

Choosing the Right Storage Driver

The choice of storage driver depends on several factors:

1. Performance: overlay2 is generally the best choice for most Linux distributions due to its performance benefits.
2. Features: If you need advanced features like snapshots and compression, btrfs or zfs might be more appropriate.
3. Compatibility: If you're using older distributions or Linux kernels, aufs might be used, but it's deprecated.
4. Storage type: If your system is configured with a particular filesystem (e.g., zfs), that might dictate the storage driver.

Conclusion

Docker storage drivers play a crucial role in how container data is managed and stored. The overlay2 driver is the most commonly used and is the recommended choice for most users, providing excellent performance and compatibility. Depending on your use case and system requirements, you might choose a different driver like aufs, btrfs, devicemapper, or zfs.