Docker ストレージドライバー

読む時間の目安: 10 分

理想を言えば、コンテナーの書き込みレイヤーへのデータの書き込みは最小とし、データ出力先には Docker ボリュームを利用します。 しかし処理内容によっては、書き込みレイヤーにデータを書き込めるようにする必要が出てきます。 これがあるからこそストレージドライバーが必要になります。

Docker supports several storage drivers, using a pluggable architecture. The storage driver controls how images and containers are stored and managed on your Docker host. After you have read the storage driver overview, the next step is to choose the best storage driver for your workloads. Use the storage driver with the best overall performance and stability in the most usual scenarios.

The Docker Engine provides the following storage drivers on Linux:

The Docker Engine has a prioritized list of which storage driver to use if no storage driver is explicitly configured, assuming that the storage driver meets the prerequisites, and automatically selects a compatible storage driver. You can see the order in the source code for Docker Engine 20.10.

Some storage drivers require you to use a specific format for the backing filesystem. If you have external requirements to use a specific backing filesystem, this may limit your choices. See Supported backing filesystems.

After you have narrowed down which storage drivers you can choose from, your choice is determined by the characteristics of your workload and the level of stability you need. See Other considerations for help in making the final decision.

Supported storage drivers per Linux distribution

Docker Desktop, and Docker in Rootless mode

Modifying the storage-driver is not supported on Docker Desktop for Mac and Docker Desktop for Windows, and only the default storage driver can be used. The comparison table below is also not applicable for Rootless mode. For the drivers available in rootless mode, see the Rootless mode documentation.

Your operating system and kernel may not support every storage driver. For instance, aufs is only supported on Ubuntu and Debian, and may require extra packages to be installed, while btrfs is only supported if your system uses btrfs as storage. In general, the following configurations work on recent versions of the Linux distribution:

¹) The overlay storage driver is deprecated, and will be removed in a future release. It is recommended that users of the overlay storage driver migrate to overlay2.

²) The devicemapper storage driver is deprecated, and will be removed in a future release. It is recommended that users of the devicemapper storage driver migrate to overlay2.

³) The aufs storage driver is deprecated, and will be removed in a future release. It is recommended that users of the aufs storage driver migrate to overlay2.

When in doubt, the best all-around configuration is to use a modern Linux distribution with a kernel that supports the overlay2 storage driver, and to use Docker volumes for write-heavy workloads instead of relying on writing data to the container’s writable layer.

The vfs storage driver is usually not the best choice, and primarily intended for debugging purposes in situations where no other storage-driver is supported. Before using the vfs storage driver, be sure to read about its performance and storage characteristics and limitations.

The recommendations in the table above are known to work for a large number of users. If you use a recommended configuration and find a reproducible issue, it is likely to be fixed very quickly. If the driver that you want to use is not recommended according to this table, you can run it at your own risk. You can and should still report any issues you run into. However, such issues have a lower priority than issues encountered when using a recommended configuration.

Depending on your Linux distribution, other storage-drivers, such as btrfs may be available. These storage drivers can have advantages for specific use-cases, but may require additional set-up or maintenance, which make them not recommended for common scenarios. Refer to the documentation for those storage drivers for details.

Supported backing filesystems

With regard to Docker, the backing filesystem is the filesystem where /var/lib/docker/ is located. Some storage drivers only work with specific backing filesystems.

Other considerations

Suitability for your workload

Among other things, each storage driver has its own performance characteristics that make it more or less suitable for different workloads. Consider the following generalizations:

• overlay2, aufs, and overlay all operate at the file level rather than the block level. This uses memory more efficiently, but the container’s writable layer may grow quite large in write-heavy workloads.
• Block-level storage drivers such as devicemapper, btrfs, and zfs perform better for write-heavy workloads (though not as well as Docker volumes).
• For lots of small writes or containers with many layers or deep filesystems, overlay may perform better than overlay2, but consumes more inodes, which can lead to inode exhaustion.
• btrfs and zfs require a lot of memory.
• zfs is a good choice for high-density workloads such as PaaS.

More information about performance, suitability, and best practices is available in the documentation for each storage driver.

Shared storage systems and the storage driver

If your enterprise uses SAN, NAS, hardware RAID, or other shared storage systems, they may provide high availability, increased performance, thin provisioning, deduplication, and compression. In many cases, Docker can work on top of these storage systems, but Docker does not closely integrate with them.

Each Docker storage driver is based on a Linux filesystem or volume manager. Be sure to follow existing best practices for operating your storage driver (filesystem or volume manager) on top of your shared storage system. For example, if using the ZFS storage driver on top of a shared storage system, be sure to follow best practices for operating ZFS filesystems on top of that specific shared storage system.

Stability

For some users, stability is more important than performance. Though Docker considers all of the storage drivers mentioned here to be stable, some are newer and are still under active development. In general, overlay2, aufs, and devicemapper are the choices with the highest stability.

Test with your own workloads

You can test Docker’s performance when running your own workloads on different storage drivers. Make sure to use equivalent hardware and workloads to match production conditions, so you can see which storage driver offers the best overall performance.

Check your current storage driver

The detailed documentation for each individual storage driver details all of the set-up steps to use a given storage driver.

To see what storage driver Docker is currently using, use docker info and look for the Storage Driver line:

$ docker info

Containers: 0
Images: 0
Storage Driver: overlay2
 Backing Filesystem: xfs
<...>

To change the storage driver, see the specific instructions for the new storage driver. Some drivers require additional configuration, including configuration to physical or logical disks on the Docker host.

Important

When you change the storage driver, any existing images and containers become inaccessible. This is because their layers cannot be used by the new storage driver. If you revert your changes, you can access the old images and containers again, but any that you pulled or created using the new driver are then inaccessible.

Related information

• About images, containers, and storage drivers
• aufs storage driver in practice
• devicemapper storage driver in practice
• overlay and overlay2 storage drivers in practice
• btrfs storage driver in practice
• zfs storage driver in practice