github.com/afein/docker@v1.8.2/daemon/graphdriver/devmapper/README.md (about)

     1  ## devicemapper - a storage backend based on Device Mapper
     2  
     3  ### Theory of operation
     4  
     5  The device mapper graphdriver uses the device mapper thin provisioning
     6  module (dm-thinp) to implement CoW snapshots. For each devicemapper
     7  graph location (typically `/var/lib/docker/devicemapper`, $graph below)
     8  a thin pool is created based on two block devices, one for data and
     9  one for metadata.  By default these block devices are created
    10  automatically by using loopback mounts of automatically created sparse
    11  files.
    12  
    13  The default loopback files used are `$graph/devicemapper/data` and
    14  `$graph/devicemapper/metadata`. Additional metadata required to map
    15  from docker entities to the corresponding devicemapper volumes is
    16  stored in the `$graph/devicemapper/json` file (encoded as Json).
    17  
    18  In order to support multiple devicemapper graphs on a system, the thin
    19  pool will be named something like: `docker-0:33-19478248-pool`, where
    20  the `0:33` part is the minor/major device nr and `19478248` is the
    21  inode number of the $graph directory.
    22  
    23  On the thin pool, docker automatically creates a base thin device,
    24  called something like `docker-0:33-19478248-base` of a fixed
    25  size. This is automatically formatted with an empty filesystem on
    26  creation. This device is the base of all docker images and
    27  containers. All base images are snapshots of this device and those
    28  images are then in turn used as snapshots for other images and
    29  eventually containers.
    30  
    31  ### Information on `docker info`
    32  
    33  As of docker-1.4.1, `docker info` when using the `devicemapper` storage driver
    34  will display something like:
    35  
    36  	$ sudo docker info
    37  	[...]
    38  	Storage Driver: devicemapper
    39  	 Pool Name: docker-253:1-17538953-pool
    40  	 Pool Blocksize: 65.54 kB
    41  	 Data file: /dev/loop4
    42  	 Metadata file: /dev/loop4
    43  	 Data Space Used: 2.536 GB
    44  	 Data Space Total: 107.4 GB
    45  	 Data Space Available: 104.8 GB
    46  	 Metadata Space Used: 7.93 MB
    47  	 Metadata Space Total: 2.147 GB
    48  	 Metadata Space Available: 2.14 GB
    49  	 Udev Sync Supported: true
    50  	 Data loop file: /home/docker/devicemapper/devicemapper/data
    51  	 Metadata loop file: /home/docker/devicemapper/devicemapper/metadata
    52  	 Library Version: 1.02.82-git (2013-10-04)
    53  	[...]
    54  
    55  #### status items
    56  
    57  Each item in the indented section under `Storage Driver: devicemapper` are
    58  status information about the driver.
    59   *  `Pool Name` name of the devicemapper pool for this driver.
    60   *  `Pool Blocksize` tells the blocksize the thin pool was initialized with. This only changes on creation.
    61   *  `Data file` blockdevice file used for the devicemapper data
    62   *  `Metadata file` blockdevice file used for the devicemapper metadata
    63   *  `Data Space Used` tells how much of `Data file` is currently used
    64   *  `Data Space Total` tells max size the `Data file`
    65   *  `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.
    66   *  `Metadata Space Used` tells how much of `Metadata file` is currently used
    67   *  `Metadata Space Total` tells max size the `Metadata file`
    68   *  `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.
    69   *  `Udev Sync Supported` tells whether devicemapper is able to sync with Udev. Should be `true`.
    70   *  `Data loop file` file attached to `Data file`, if loopback device is used
    71   *  `Metadata loop file` file attached to `Metadata file`, if loopback device is used
    72   *  `Library Version` from the libdevmapper used
    73  
    74  ### About the devicemapper options
    75  
    76  The devicemapper backend supports some options that you can specify
    77  when starting the docker daemon using the `--storage-opt` flags.
    78  This uses the `dm` prefix and would be used something like `docker -d --storage-opt dm.foo=bar`.
    79  
    80  These options are currently documented both in [the man
    81  page](../../../man/docker.1.md) and in [the online
    82  documentation](https://docs.docker.com/reference/commandline/daemon/#docker-
    83  execdriver-option).  If you add an options, update both the `man` page and the
    84  documentation.