143 lines
4.7 KiB
Markdown
143 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. For each devicemapper
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graph location (typically `/var/lib/docker/devicemapper`, $graph below)
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a thin pool is created based on two block devices, one for data and
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one for metadata. By default these block devices are created
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automatically by using loopback mounts of automatically creates sparse
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files.
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The default loopback files used are `$graph/devicemapper/data` and
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`$graph/devicemapper/metadata`. Additional metadata required to map
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from docker entities to the corresponding devicemapper volumes is
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stored in the `$graph/devicemapper/json` 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 `0:30` part is the minor/major device nr and `19478248` is the
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inode number of the $graph 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 formated on creation and contains just an
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empty filesystem. 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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### 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 somthing like `docker -d --storage-opt dm.foo=bar`.
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Here is the list of supported options:
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* `dm.basesize`
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Specifies the size to use when creating the base device, which
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limits the size of images and containers. The default value is
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10G. Note, thin devices are inherently "sparse", so a 10G device
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which is mostly empty doesn't use 10 GB of space on the
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pool. However, the filesystem will use more space for the empty
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case the larger the device is.
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Example use:
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``docker -d --storage-opt dm.basesize=20G``
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* `dm.loopdatasize`
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Specifies the size to use when creating the loopback file for the
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"data" device which is used for the thin pool. The default size is
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100G. Note that the file is sparse, so it will not initially take
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up this much space.
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Example use:
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``docker -d --storage-opt dm.loopdatasize=200G``
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* `dm.loopmetadatasize`
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Specifies the size to use when creating the loopback file for the
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"metadadata" device which is used for the thin pool. The default size is
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2G. Note that the file is sparse, so it will not initially take
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up this much space.
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Example use:
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``docker -d --storage-opt dm.loopmetadatasize=4G``
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* `dm.fs`
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Specifies the filesystem type to use for the base device. The supported
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options are "ext4" and "xfs". The default is "ext4"
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Example use:
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``docker -d --storage-opt dm.fs=xfs``
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* `dm.mkfsarg`
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Specifies extra mkfs arguments to be used when creating the base device.
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Example use:
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``docker -d --storage-opt "dm.mkfsarg=-O ^has_journal"``
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* `dm.mountopt`
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Specifies extra mount options used when mounting the thin devices.
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Example use:
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``docker -d --storage-opt dm.mountopt=nodiscard``
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* `dm.datadev`
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Specifies a custom blockdevice to use for data for the thin pool.
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If using a block device for device mapper storage, ideally both
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datadev and metadatadev should be specified to completely avoid
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using the loopback device.
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Example use:
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``docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1``
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* `dm.metadatadev`
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Specifies a custom blockdevice to use for metadata for the thin
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pool.
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For best performance the metadata should be on a different spindle
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than the data, or even better on an SSD.
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If setting up a new metadata pool it is required to be valid. This
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can be achieved by zeroing the first 4k to indicate empty
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metadata, like this:
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``dd if=/dev/zero of=$metadata_dev bs=4096 count=1```
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Example use:
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``docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1``
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* `dm.blkdiscard`
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Enables or disables the use of blkdiscard when removing
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devicemapper devices. This is enabled by default (only) if using
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loopback devices and is required to res-parsify the loopback file
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on image/container removal.
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Disabling this on loopback can lead to *much* faster container
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removal times, but will make the space used in /var/lib/docker
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directory not be returned to the system for other use when
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containers are removed.
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Example use:
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``docker -d --storage-opt dm.blkdiscard=false``
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