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. Alternatively, the device mapper graphdriver can setup a block device to handle this for you via the
--storage-opt dm.directlvm_device 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/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
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.
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) [...]
Each item in the indented section under
Storage Driver: devicemapper are status information about the driver.
Pool Namename of the devicemapper pool for this driver.
Pool Blocksizetells the blocksize the thin pool was initialized with. This only changes on creation.
Base Device Sizetells the maximum size of a container and image
Data fileblockdevice file used for the devicemapper data
Metadata fileblockdevice file used for the devicemapper metadata
Data Space Usedtells how much of
Data fileis currently used
Data Space Totaltells max size the
Data Space Availabletells 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 Usedtells how much of
Metadata fileis currently used
Metadata Space Totaltells max size the
Metadata Space Availabletells 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 Supportedtells whether devicemapper is able to sync with Udev. Should be
Data loop filefile attached to
Data file, if loopback device is used
Metadata loop filefile attached to
Metadata file, if loopback device is used
Library Versionfrom the libdevmapper used
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
dockerd --storage-opt dm.foo=bar.