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.
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
Metadata Space Used: 7.93 MB
Metadata Space Total: 2.147 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
Metadata Space Used
tells how much of Metadata file
is currently used
Metadata Space Total
tells max size the Metadata file
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 it's 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