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moby--moby/daemon/graphdriver/devmapper
Derek 9bbed5ab4c change to lazy Unmount
syscall.Unmount failed sometimes when user interrupted exporting,
for example a Ctrl-C, or pipe to commands which closed the pipe early,
like "docker export <container_name> | file -"; this syscall.Unmount
could sometimes return EBUSY and didn't actually umount the filesystem;
which would cause a following export command fail to mount;
change to lazy Unmount with MNT_DETACH can fix the problem, this is
the same behavior as in Shutdown;

```text
time="2015-01-03T21:27:26Z" level=error msg="Warning: error unmounting device
34a3e77cdbca17ceffd0636aee0415bb412996adb12360bfe2585ce30467fa8e: device or resource busy"
```

```
$ docker export thirsty_ardinghelli | file -
/dev/stdin: POSIX tar archive
time="2015-01-03T21:58:17Z" level=fatal msg="write /dev/stdout: broken pipe"
$ docker export thirsty_ardinghelli
time="2015-01-03T21:54:33Z" level=fatal msg="Error: thirsty_ardinghelli: Error getting container
34a3e77cdbca17ceffd0636aee0415bb412996adb12360bfe2585ce30467fa8e from driver devicemapper:
Error mounting '/dev/mapper/docker-253:0-3148372-34a3e77cdbca17ceffd0636aee0415bb412996adb12360bfe2585ce30467fa8e'
on '/var/lib/docker/devicemapper/mnt/34a3e77cdbca17ceffd0636aee0415bb412996adb12360bfe2585ce30467fa8e': device or resource busy"
```

Signed-off-by: Derek Che <drc@yahoo-inc.com>
2015-01-04 00:10:48 +00:00
..
deviceset.go change to lazy Unmount 2015-01-04 00:10:48 +00:00
devmapper_doc.go Rename runtime/* to daemon/* 2014-04-17 14:43:01 -07:00
devmapper_test.go Mknod more loopbacks for devmapper 2014-11-21 16:20:35 -08:00
driver.go Fix to avoid a compilation error of size_test.go with GCCGO due to float to int truncation 2014-11-25 18:12:22 +09:00
MAINTAINERS pkg/devicemapper: missed MAINTAINERS on split 2014-11-14 11:12:23 -05:00
mount.go Replace "amd64" build tags with "cgo" as appropriate, and remove where unnecessary 2014-08-06 17:20:21 -06:00
README.md devmapper: Add option for specifying an lvm2 created thin-pool device 2014-11-12 21:03:04 -05:00

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.

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 res-parsify 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