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97 lines
4.7 KiB
Markdown
97 lines
4.7 KiB
Markdown
## devicemapper - a storage backend based on Device Mapper
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### Theory of operation
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The device mapper graphdriver uses the device mapper thin provisioning
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module (dm-thinp) to implement CoW snapshots. The preferred model is
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to have a thin pool reserved outside of Docker and passed to the
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daemon via the `--storage-opt dm.thinpooldev` option.
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As a fallback if no thin pool is provided, loopback files will be
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created. Loopback is very slow, but can be used without any
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pre-configuration of storage. It is strongly recommended that you do
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not use loopback in production. Ensure your Docker daemon has a
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`--storage-opt dm.thinpooldev` argument provided.
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In loopback, a thin pool is created at `/var/lib/docker/devicemapper`
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(devicemapper graph location) based on two block devices, one for
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data and one for metadata. By default these block devices are created
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automatically by using loopback mounts of automatically created sparse
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files.
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The default loopback files used are
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`/var/lib/docker/devicemapper/devicemapper/data` and
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`/var/lib/docker/devicemapper/devicemapper/metadata`. Additional metadata
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required to map from docker entities to the corresponding devicemapper
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volumes is stored in the `/var/lib/docker/devicemapper/devicemapper/json`
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file (encoded as Json).
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In order to support multiple devicemapper graphs on a system, the thin
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pool will be named something like: `docker-0:33-19478248-pool`, where
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the `0:33` part is the minor/major device nr and `19478248` is the
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inode number of the `/var/lib/docker/devicemapper` directory.
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On the thin pool, docker automatically creates a base thin device,
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called something like `docker-0:33-19478248-base` of a fixed
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size. This is automatically formatted with an empty filesystem on
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creation. This device is the base of all docker images and
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containers. All base images are snapshots of this device and those
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images are then in turn used as snapshots for other images and
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eventually containers.
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### Information on `docker info`
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As of docker-1.4.1, `docker info` when using the `devicemapper` storage driver
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will display something like:
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$ sudo docker info
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[...]
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Storage Driver: devicemapper
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Pool Name: docker-253:1-17538953-pool
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Pool Blocksize: 65.54 kB
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Base Device Size: 107.4 GB
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Data file: /dev/loop4
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Metadata file: /dev/loop4
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Data Space Used: 2.536 GB
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Data Space Total: 107.4 GB
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Data Space Available: 104.8 GB
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Metadata Space Used: 7.93 MB
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Metadata Space Total: 2.147 GB
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Metadata Space Available: 2.14 GB
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Udev Sync Supported: true
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Data loop file: /home/docker/devicemapper/devicemapper/data
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Metadata loop file: /home/docker/devicemapper/devicemapper/metadata
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Library Version: 1.02.82-git (2013-10-04)
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[...]
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#### status items
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Each item in the indented section under `Storage Driver: devicemapper` are
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status information about the driver.
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* `Pool Name` name of the devicemapper pool for this driver.
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* `Pool Blocksize` tells the blocksize the thin pool was initialized with. This only changes on creation.
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* `Base Device Size` tells the maximum size of a container and image
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* `Data file` blockdevice file used for the devicemapper data
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* `Metadata file` blockdevice file used for the devicemapper metadata
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* `Data Space Used` tells how much of `Data file` is currently used
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* `Data Space Total` tells max size the `Data file`
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* `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.
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* `Metadata Space Used` tells how much of `Metadata file` is currently used
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* `Metadata Space Total` tells max size the `Metadata file`
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* `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.
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* `Udev Sync Supported` tells whether devicemapper is able to sync with Udev. Should be `true`.
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* `Data loop file` file attached to `Data file`, if loopback device is used
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* `Metadata loop file` file attached to `Metadata file`, if loopback device is used
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* `Library Version` from the libdevmapper used
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### About the devicemapper options
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The devicemapper backend supports some options that you can specify
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when starting the docker daemon using the `--storage-opt` flags.
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This uses the `dm` prefix and would be used something like `docker daemon --storage-opt dm.foo=bar`.
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These options are currently documented both in [the man
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page](../../../man/docker.1.md) and in [the online
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documentation](https://docs.docker.com/reference/commandline/daemon/#docker-
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execdriver-option). If you add an options, update both the `man` page and the
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documentation.
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