Select a storage driver

This page describes Docker’s storage driver feature. It lists the storage drivers that Docker supports and the basic commands associated with managing them. Finally, this page provides guidance on choosing a storage driver.

The material on this page is intended for readers who already have an understanding of the storage driver technology.

A pluggable storage driver architecture

Docker has a pluggable storage driver architecture. This gives you the flexibility to “plug in” the storage driver that is best for your environment and use-case. Each Docker storage driver is based on a Linux filesystem or volume manager. Further, each storage driver is free to implement the management of image layers and the container layer in its own unique way. This means some storage drivers perform better than others in different circumstances.

Once you decide which driver is best, you set this driver on the Docker daemon at start time. As a result, the Docker daemon can only run one storage driver, and all containers created by that daemon instance use the same storage driver. The table below shows the supported storage driver technologies and their driver names:

To find out which storage driver is set on the daemon, you use the docker info command:

$ docker info

Containers: 0
Images: 0
Storage Driver: overlay
 Backing Filesystem: extfs
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.19.0-15-generic
Operating System: Ubuntu 15.04
... output truncated ...

The info subcommand reveals that the Docker daemon is using the overlay storage driver with a Backing Filesystem value of extfs. The extfs value means that the overlay storage driver is operating on top of an existing (ext) filesystem. The backing filesystem refers to the filesystem that was used to create the Docker host’s local storage area under /var/lib/docker.

Which storage driver you use, in part, depends on the backing filesystem you plan to use for your Docker host’s local storage area. Some storage drivers can operate on top of different backing filesystems. However, other storage drivers require the backing filesystem to be the same as the storage driver. For example, the btrfs storage driver on a Btrfs backing filesystem. The following table lists each storage driver and whether it must match the host’s backing filesystem:

Note “Disabled on” means some storage drivers can not run over certain backing filesystem.

You can set the storage driver by passing the --storage-driver=<name> option to the dockerd command line, or by setting the option on the DOCKER_OPTS line in the /etc/default/docker file.

The following command shows how to start the Docker daemon with the devicemapper storage driver using the dockerd command:

$ dockerd --storage-driver=devicemapper &

$ docker info

Containers: 0
Images: 0
Storage Driver: devicemapper
 Pool Name: docker-252:0-147544-pool
 Pool Blocksize: 65.54 kB
 Backing Filesystem: extfs
 Data file: /dev/loop0
 Metadata file: /dev/loop1
 Data Space Used: 1.821 GB
 Data Space Total: 107.4 GB
 Data Space Available: 3.174 GB
 Metadata Space Used: 1.479 MB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.146 GB
 Thin Pool Minimum Free Space: 10.74 GB
 Udev Sync Supported: true
 Deferred Removal Enabled: false
 Data loop file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.90 (2014-09-01)
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.19.0-15-generic
Operating System: Ubuntu 15.04
<output truncated>

Your choice of storage driver can affect the performance of your containerized applications. So it’s important to understand the different storage driver options available and select the right one for your application. Later, in this page you’ll find some advice for choosing an appropriate driver.

Shared storage systems and the storage driver

Many enterprises consume storage from shared storage systems such as SAN and NAS arrays. These often provide increased performance and availability, as well as advanced features such as thin provisioning, deduplication and compression.

The Docker storage driver and data volumes can both operate on top of storage provided by shared storage systems. This allows Docker to leverage the increased performance and availability these systems provide. However, Docker does not integrate with these underlying systems.

Remember that 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 XYZ shared storage system, be sure to follow best practices for operating ZFS filesystems on top of XYZ shared storage system.

Which storage driver should you choose?

Several factors influence the selection of a storage driver. However, these two facts must be kept in mind:

1. No single driver is well suited to every use-case
2. Storage drivers are improving and evolving all of the time

With these factors in mind, the following points, coupled with the table below, should provide some guidance.

Stability

For the most stable and hassle-free Docker experience, you should consider the following:

• Use the default storage driver for your distribution. When Docker installs, it chooses a default storage driver based on the configuration of your system. Stability is an important factor influencing which storage driver is used by default. Straying from this default may increase your chances of encountering bugs and nuances.
• Follow the configuration specified on the CS Engine compatibility matrix. The CS Engine is the commercially supported version of the Docker Engine. It’s code-base is identical to the open source Engine, but it has a limited set of supported configurations. These supported configurations use the most stable and mature storage drivers. Straying from these configurations may also increase your chances of encountering bugs and nuances.

Experience and expertise

Choose a storage driver that you and your team/organization have experience with. For example, if you use RHEL or one of its downstream forks, you may already have experience with LVM and Device Mapper. If so, you may wish to use the devicemapper driver.

If you do not feel you have expertise with any of the storage drivers supported by Docker, and you want an easy-to-use stable Docker experience, you should consider using the default driver installed by your distribution’s Docker package.

Future-proofing

Many people consider OverlayFS as the future of the Docker storage driver. However, it is less mature, and potentially less stable than some of the more mature drivers such as aufs and devicemapper. For this reason, you should use the OverlayFS driver with caution and expect to encounter more bugs and nuances than if you were using a more mature driver.

The following diagram lists each storage driver and provides insight into some of their pros and cons. When selecting which storage driver to use, consider the guidance offered by the table below along with the points mentioned above.

Overlay vs Overlay2

OverlayFS has 2 storage drivers which both make use of the same OverlayFS technology but with different implementations and incompatible on disk storage. Since the storage is incompatible, switching between the two will require re-creating all image content. The overlay driver is the original implementation and the only option in Docker 1.11 and before. The overlay driver has known limitations with inode exhaustion and commit performance. The overlay2 driver addresses this limitation, but is only compatible with Linux kernel 4.0 and later. For users on a pre-4.0 kernel or with an existing overlay graph, it is recommended to stay on overlay. For users with at least a 4.0 kernel and no existing or required overlay graph data, then overlay2 may be used.

Note overlay2 graph data will not interfere with overlay graph data. However when switching to overlay2, the user is responsible for removing overlay graph data to avoid storage duplication.

Related information

• Understand images, containers, and storage drivers
• AUFS storage driver in practice
• Btrfs storage driver in practice
• Device Mapper storage driver in practice

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