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README.md |
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 imageData file
blockdevice file used for the devicemapper dataMetadata file
blockdevice file used for the devicemapper metadataData Space Used
tells how much ofData file
is currently usedData Space Total
tells max size theData file
Data Space Available
tells how much free space there is in theData 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 ofMetadata file
is currently usedMetadata Space Total
tells max size theMetadata file
Metadata Space Available
tells how much free space there is in theMetadata 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 betrue
.Data loop file
file attached toData file
, if loopback device is usedMetadata loop file
file attached toMetadata file
, if loopback device is usedLibrary 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](https://docs.docker.com/reference/commandline/daemon/#docker-
execdriver-option). If you add an options, update both the man
page and the
documentation.