github.com/demonoid81/moby@v0.0.0-20200517203328-62dd8e17c460/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. The preferred model is 7 to have a thin pool reserved outside of Docker and passed to the 8 daemon via the `--storage-opt dm.thinpooldev` option. Alternatively, 9 the device mapper graphdriver can setup a block device to handle this 10 for you via the `--storage-opt dm.directlvm_device` option. 11 12 As a fallback if no thin pool is provided, loopback files will be 13 created. Loopback is very slow, but can be used without any 14 pre-configuration of storage. It is strongly recommended that you do 15 not use loopback in production. Ensure your Docker daemon has a 16 `--storage-opt dm.thinpooldev` argument provided. 17 18 In loopback, a thin pool is created at `/var/lib/docker/devicemapper` 19 (devicemapper graph location) based on two block devices, one for 20 data and one for metadata. By default these block devices are created 21 automatically by using loopback mounts of automatically created sparse 22 files. 23 24 The default loopback files used are 25 `/var/lib/docker/devicemapper/devicemapper/data` and 26 `/var/lib/docker/devicemapper/devicemapper/metadata`. Additional metadata 27 required to map from docker entities to the corresponding devicemapper 28 volumes is stored in the `/var/lib/docker/devicemapper/devicemapper/json` 29 file (encoded as Json). 30 31 In order to support multiple devicemapper graphs on a system, the thin 32 pool will be named something like: `docker-0:33-19478248-pool`, where 33 the `0:33` part is the minor/major device nr and `19478248` is the 34 inode number of the `/var/lib/docker/devicemapper` directory. 35 36 On the thin pool, docker automatically creates a base thin device, 37 called something like `docker-0:33-19478248-base` of a fixed 38 size. This is automatically formatted with an empty filesystem on 39 creation. This device is the base of all docker images and 40 containers. All base images are snapshots of this device and those 41 images are then in turn used as snapshots for other images and 42 eventually containers. 43 44 ## Information on `docker info` 45 46 As of docker-1.4.1, `docker info` when using the `devicemapper` storage driver 47 will display something like: 48 49 $ sudo docker info 50 [...] 51 Storage Driver: devicemapper 52 Pool Name: docker-253:1-17538953-pool 53 Pool Blocksize: 65.54 kB 54 Base Device Size: 107.4 GB 55 Data file: /dev/loop4 56 Metadata file: /dev/loop4 57 Data Space Used: 2.536 GB 58 Data Space Total: 107.4 GB 59 Data Space Available: 104.8 GB 60 Metadata Space Used: 7.93 MB 61 Metadata Space Total: 2.147 GB 62 Metadata Space Available: 2.14 GB 63 Udev Sync Supported: true 64 Data loop file: /home/docker/devicemapper/devicemapper/data 65 Metadata loop file: /home/docker/devicemapper/devicemapper/metadata 66 Library Version: 1.02.82-git (2013-10-04) 67 [...] 68 69 ### status items 70 71 Each item in the indented section under `Storage Driver: devicemapper` are 72 status information about the driver. 73 * `Pool Name` name of the devicemapper pool for this driver. 74 * `Pool Blocksize` tells the blocksize the thin pool was initialized with. This only changes on creation. 75 * `Base Device Size` tells the maximum size of a container and image 76 * `Data file` blockdevice file used for the devicemapper data 77 * `Metadata file` blockdevice file used for the devicemapper metadata 78 * `Data Space Used` tells how much of `Data file` is currently used 79 * `Data Space Total` tells max size the `Data file` 80 * `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. 81 * `Metadata Space Used` tells how much of `Metadata file` is currently used 82 * `Metadata Space Total` tells max size the `Metadata file` 83 * `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. 84 * `Udev Sync Supported` tells whether devicemapper is able to sync with Udev. Should be `true`. 85 * `Data loop file` file attached to `Data file`, if loopback device is used 86 * `Metadata loop file` file attached to `Metadata file`, if loopback device is used 87 * `Library Version` from the libdevmapper used 88 89 ## About the devicemapper options 90 91 The devicemapper backend supports some options that you can specify 92 when starting the docker daemon using the `--storage-opt` flags. 93 This uses the `dm` prefix and would be used something like `dockerd --storage-opt dm.foo=bar`. 94 95 These options are currently documented both in [the man 96 page](../../../man/docker.1.md) and in [the online 97 documentation](https://docs.docker.com/engine/reference/commandline/dockerd/#/storage-driver-options). 98 If you add an options, update both the `man` page and the documentation.