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.
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) [...]
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 dataMetadata file
blockdevice file used for the devicemapper metadataData Space Used
tells how much of Data file
is currently usedData 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 usedMetadata 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 usedMetadata loop file
file attached to Metadata file
, if loopback device is usedLibrary Version
from the libdevmapper usedThe 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
dm.override_udev_sync_check
Overrides the udev
synchronization checks between devicemapper
and udev
. udev
is the device manager for the Linux kernel.
To view the udev
sync support of a Docker daemon that is using the devicemapper
driver, run:
$ docker info
[...] Udev Sync Supported: true [...]
When udev
sync support is true
, then devicemapper
and udev can coordinate the activation and deactivation of devices for containers.
When udev
sync support is false
, a race condition occurs between thedevicemapper
and udev
during create and cleanup. The race condition results in errors and failures. (For information on these failures, see docker#4036)
To allow the docker
daemon to start, regardless of udev
sync not being supported, set dm.override_udev_sync_check
to true:
$ docker -d --storage-opt dm.override_udev_sync_check=true
When this value is true
, the devicemapper
continues and simply warns you the errors are happening.
Note: The ideal is to pursue a
docker
daemon and environment that does support synchronizing withudev
. For further discussion on this topic, see docker#4036. Otherwise, set this flag for migrating existing Docker daemons to a daemon with a supported environment.
dm.use_deferred_removal
Enables use of deferred device removal if libdm and kernel driver support the mechanism.
Deferred device removal means that if device is busy when devices is being removed/deactivated, then a deferred removal is scheduled on device. And devices automatically goes away when last user of device exits.
For example, when contianer exits, its associated thin device is removed. If that devices has leaked into some other mount namespace can can't be removed now, container exit will still be successful and this option will just schedule device for deferred removal and will not wait in a loop trying to remove a busy device.
Example use:
docker -d --storage-opt dm.use_deferred_removal=true