devmapper

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 creates 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 0:30 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 formated on creation and contains just an empty filesystem. 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.

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 somthing 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.

  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.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.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 res-parsify 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