devicemapper - a storage backend based on Device Mapper

Theory of operation

The device mapper graphdriver uses the device mapper thin provisioning module (dm-thinp) to implement CoW snapshots. The preferred model is to have a thin pool reserved outside of Docker and passed to the daemon via the --storage-opt dm.thinpooldev option.

As a fallback if no thin pool is provided, loopback files will be created. Loopback is very slow, but can be used without any pre-configuration of storage. It is strongly recommended that you do not use loopback in production. Ensure your Docker daemon has a --storage-opt dm.thinpooldev argument provided.

In loopback, a thin pool is created at /var/lib/docker/devicemapper (devicemapper graph location) based on two block devices, one for data and one for metadata. By default these block devices are created automatically by using loopback mounts of automatically created sparse files.

The default loopback files used are /var/lib/docker/devicemapper/devicemapper/data and /var/lib/docker/devicemapper/devicemapper/metadata. Additional metadata required to map from docker entities to the corresponding devicemapper volumes is stored in the /var/lib/docker/devicemapper/devicemapper/json file (encoded as Json).

In order to support multiple devicemapper graphs on a system, the thin pool will be named something like: docker-0:33-19478248-pool, where the 0:33 part is the minor/major device nr and 19478248 is the inode number of the /var/lib/docker/devicemapper directory.

On the thin pool, docker automatically creates a base thin device, called something like docker-0:33-19478248-base of a fixed size. This is automatically formatted with an empty filesystem on creation. This device is the base of all docker images and containers. All base images are snapshots of this device and those images are then in turn used as snapshots for other images and eventually containers.

Information on docker info

As of docker-1.4.1, docker info when using the devicemapper storage driver will display something like:

$ sudo docker info
[...]
Storage Driver: devicemapper
 Pool Name: docker-253:1-17538953-pool
 Pool Blocksize: 65.54 kB
 Base Device Size: 107.4 GB
 Data file: /dev/loop4
 Metadata file: /dev/loop4
 Data Space Used: 2.536 GB
 Data Space Total: 107.4 GB
 Data Space Available: 104.8 GB
 Metadata Space Used: 7.93 MB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.14 GB
 Udev Sync Supported: true
 Data loop file: /home/docker/devicemapper/devicemapper/data
 Metadata loop file: /home/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.82-git (2013-10-04)
[...]

status items

Each item in the indented section under Storage Driver: devicemapper are status information about the driver.

• Pool Name name of the devicemapper pool for this driver.
• Pool Blocksize tells the blocksize the thin pool was initialized with. This only changes on creation.
• Base Device Size tells the maximum size of a container and image
• Data file blockdevice file used for the devicemapper data
• Metadata file blockdevice file used for the devicemapper metadata
• Data Space Used tells how much of Data file is currently used
• Data Space Total tells max size the Data file
• Data Space Available tells how much free space there is in the Data file. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
• Metadata Space Used tells how much of Metadata file is currently used
• Metadata Space Total tells max size the Metadata file
• Metadata Space Available tells how much free space there is in the Metadata file. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
• Udev Sync Supported tells whether devicemapper is able to sync with Udev. Should be true.
• Data loop file file attached to Data file, if loopback device is used
• Metadata loop file file attached to Metadata file, if loopback device is used
• Library Version from the libdevmapper used

About the devicemapper options

The devicemapper backend supports some options that you can specify when starting the docker daemon using the --storage-opt flags. This uses the dm prefix and would be used something like docker daemon --storage-opt dm.foo=bar.

These options are currently documented both in the man page and in the online documentation. If you add an options, update both the man page and the documentation.