docs/devmapper at 33ed76b0f79e639e9ad3d35ee39afa3d9216c500 - docs

History

Kir Kolyshkin a83a769347 Simplify and fix os.MkdirAll() usage TL;DR: check for IsExist(err) after a failed MkdirAll() is both redundant and wrong -- so two reasons to remove it. Quoting MkdirAll documentation: > MkdirAll creates a directory named path, along with any necessary > parents, and returns nil, or else returns an error. If path > is already a directory, MkdirAll does nothing and returns nil. This means two things: 1. If a directory to be created already exists, no error is returned. 2. If the error returned is IsExist (EEXIST), it means there exists a non-directory with the same name as MkdirAll need to use for directory. Example: we want to MkdirAll("a/b"), but file "a" (or "a/b") already exists, so MkdirAll fails. The above is a theory, based on quoted documentation and my UNIX knowledge. 3. In practice, though, current MkdirAll implementation [1] returns ENOTDIR in most of cases described in #2, with the exception when there is a race between MkdirAll and someone else creating the last component of MkdirAll argument as a file. In this very case MkdirAll() will indeed return EEXIST. Because of #1, IsExist check after MkdirAll is not needed. Because of #2 and #3, ignoring IsExist error is just plain wrong, as directory we require is not created. It's cleaner to report the error now. Note this error is all over the tree, I guess due to copy-paste, or trying to follow the same usage pattern as for Mkdir(), or some not quite correct examples on the Internet. [v2: a separate aufs commit is merged into this one] [1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go Signed-off-by: Kir Kolyshkin <kir@openvz.org>		2015-07-30 11:48:08 -07:00
..
README.md	Closes #13323 and carries	2015-06-29 06:18:41 -07:00
deviceset.go	Simplify and fix os.MkdirAll() usage	2015-07-30 11:48:08 -07:00
devmapper_doc.go	…
devmapper_test.go	daemon/graphdriver/devmapper/ fix lint errors/warnings	2015-07-29 01:43:34 +00:00
driver.go	Simplify and fix os.MkdirAll() usage	2015-07-30 11:48:08 -07:00
mount.go	daemon/graphdriver/devmapper/ fix lint errors/warnings	2015-07-29 01:43:34 +00:00

README.md

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.

Information on `docker info`

As of docker-1.4.1, docker info when using the devicemapper storage driver will display something like:

$ sudo docker info
[...]
Storage Driver: devicemapper
 Pool Name: docker-253:1-17538953-pool
 Pool Blocksize: 65.54 kB
 Data file: /dev/loop4
 Metadata file: /dev/loop4
 Data Space Used: 2.536 GB
 Data Space Total: 107.4 GB
 Data Space Available: 104.8 GB
 Metadata Space Used: 7.93 MB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.14 GB
 Udev Sync Supported: true
 Data loop file: /home/docker/devicemapper/devicemapper/data
 Metadata loop file: /home/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.82-git (2013-10-04)
[...]

status items

Each item in the indented section under Storage Driver: devicemapper are status information about the driver.

Pool Name name of the devicemapper pool for this driver.
Pool Blocksize tells the blocksize the thin pool was initialized with. This only changes on creation.
Data file blockdevice file used for the devicemapper data
Metadata file blockdevice file used for the devicemapper metadata
Data Space Used tells how much of Data file is currently used
Data Space Total tells max size the Data file
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.
Metadata Space Used tells how much of Metadata file is currently used
Metadata Space Total tells max size the Metadata file
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.
Udev Sync Supported tells whether devicemapper is able to sync with Udev. Should be true.
Data loop file file attached to Data file, if loopback device is used
Metadata loop file file attached to Metadata file, if loopback device is used
Library Version from the libdevmapper used

About the devicemapper 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.

These options are currently documented both in the man page and in [the online documentation](https://docs.docker.com/reference/commandline/daemon/#docker- execdriver-option). If you add an options, update both the man page and the documentation.