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. For each devicemapper graph location (typically /var/lib/docker/devicemapper, $graph below) a thin pool is created 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 $graph/devicemapper/data and $graph/devicemapper/metadata. Additional metadata required to map from docker entities to the corresponding devicemapper volumes is stored in the $graph/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 $graph 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
 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.
• 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

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 -d --storage-opt dm.foo=bar.

Here is the list of supported options:

• dm.basesize

  Specifies the size to use when creating the base device, which limits the size of images and containers. The default value is 10G. Note, thin devices are inherently “sparse”, so a 10G device which is mostly empty doesn't use 10 GB of space on the pool. However, the filesystem will use more space for the empty case the larger the device is. Warning: This value affects the system-wide “base” empty filesystem that may already be initialized and inherited by pulled images. Typically, a change to this value will require additional steps to take effect: 1) stop docker -d, 2) rm -rf /var/lib/docker, 3) start docker -d.

  Example use:

  docker -d --storage-opt dm.basesize=20G

• dm.loopdatasize

  Specifies the size to use when creating the loopback file for the “data” device which is used for the thin pool. The default size is 100G. Note that the file is sparse, so it will not initially take up this much space.

  Example use:

  docker -d --storage-opt dm.loopdatasize=200G

• dm.loopmetadatasize

  Specifies the size to use when creating the loopback file for the “metadadata” device which is used for the thin pool. The default size is 2G. Note that the file is sparse, so it will not initially take up this much space.

  Example use:

  docker -d --storage-opt dm.loopmetadatasize=4G

• dm.fs

  Specifies the filesystem type to use for the base device. The supported options are “ext4” and “xfs”. The default is “ext4”

  Example use:

  docker -d --storage-opt dm.fs=xfs

• dm.mkfsarg

  Specifies extra mkfs arguments to be used when creating the base device.

  Example use:

  docker -d --storage-opt "dm.mkfsarg=-O ^has_journal"

• dm.mountopt

  Specifies extra mount options used when mounting the thin devices.

  Example use:

  docker -d --storage-opt dm.mountopt=nodiscard

• dm.thinpooldev

  Specifies a custom blockdevice to use for the thin pool.

  If using a block device for device mapper storage, ideally lvm2 would be used to create/manage the thin-pool volume that is then handed to docker to exclusively create/manage the thin and thin snapshot volumes needed for its containers. Managing the thin-pool outside of docker makes for the most feature-rich method of having docker utilize device mapper thin provisioning as the backing storage for docker's containers. lvm2-based thin-pool management feature highlights include: automatic or interactive thin-pool resize support, dynamically change thin-pool features, automatic thinp metadata checking when lvm2 activates the thin-pool, etc.

  Example use:

  docker -d --storage-opt dm.thinpooldev=/dev/mapper/thin-pool

• dm.datadev

  Specifies a custom blockdevice to use for data for the thin pool.

  If using a block device for device mapper storage, ideally both datadev and metadatadev should be specified to completely avoid using the loopback device.

  Example use:

  docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1

• dm.metadatadev

  Specifies a custom blockdevice to use for metadata for the thin pool.

  For best performance the metadata should be on a different spindle than the data, or even better on an SSD.

  If setting up a new metadata pool it is required to be valid. This can be achieved by zeroing the first 4k to indicate empty metadata, like this:

  ``dd if=/dev/zero of=$metadata_dev bs=4096 count=1```

  Example use:

  docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1

• dm.blocksize

  Specifies a custom blocksize to use for the thin pool. The default blocksize is 64K.

  Example use:

  docker -d --storage-opt dm.blocksize=512K

• dm.blkdiscard

  Enables or disables the use of blkdiscard when removing devicemapper devices. This is enabled by default (only) if using loopback devices and is required to resparsify the loopback file on image/container removal.

  Disabling this on loopback can lead to much faster container removal times, but will make the space used in /var/lib/docker directory not be returned to the system for other use when containers are removed.

  Example use:

  docker -d --storage-opt dm.blkdiscard=false