Add option to auto-configure blkdev for devmapper

Instead of forcing users to manually configure a block device to use
with devmapper, this gives the user the option to let the devmapper
driver configure a device for them.

Adds several new options to the devmapper storage-opts:

- dm.directlvm_device="" - path to the block device to configure for
  direct-lvm
- dm.thinp_percent=95 - sets the percentage of space to use for
  storage from the passed in block device
- dm.thinp_metapercent=1 - sets the percentage of space to for metadata
  storage from the passed in block device
- dm.thinp_autoextend_threshold=80 - sets the threshold for when `lvm`
  should automatically extend the thin pool as a percentage of the total
  storage space
- dm.thinp_autoextend_percent=20 - sets the percentage to increase the
  thin pool by when an autoextend is triggered.

Defaults are taken from
[here](https://docs.docker.com/engine/userguide/storagedriver/device-mapper-driver/#/configure-direct-lvm-mode-for-production)

The only option that is required is `dm.directlvm_device` for docker to
set everything up.

Changes to these settings are not currently supported and will error
out.
Future work could support allowing changes to these values.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

docs/reference/commandline/dockerd.md

@@ -343,6 +343,60 @@ not use loopback in production. Ensure your Engine daemon has a
 $ sudo dockerd --storage-opt dm.thinpooldev=/dev/mapper/thin-pool
 ```
 
+##### `dm.directlvm_device`
+
+As an alternative to providing a thin pool as above, Docker can setup a block
+device for you.
+
+###### Example:
+
+```bash
+$ sudo dockerd --storage-opt dm.directlvm_device=/dev/xvdf
+```
+
+##### `dm.thinp_percent`
+
+Sets the percentage of passed in block device to use for storage.
+
+###### Example:
+
+```bash
+$ sudo dockerd --storage-opt dm.thinp_percent=95
+```
+
+##### `dm.thinp_metapercent`
+
+Sets the percentage of the passed in block device to use for metadata storage.
+
+###### Example:
+
+```bash
+$ sudo dockerd --storage-opt dm.thinp_metapercent=1
+```
+
+##### `dm.thinp_autoextend_threshold`
+
+Sets the value of the percentage of space used before `lvm` attempts to
+autoextend the available space [100 = disabled]
+
+###### Example:
+
+```bash
+$ sudo dockerd --storage-opt dm.thinp_autoextend_threshold=80
+```
+
+##### `dm.thinp_autoextend_percent`
+
+Sets the value percentage value to increase the thin pool by when when `lvm`
+attempts to autoextend the available space [100 = disabled]
+
+###### Example:
+
+```bash
+$ sudo dockerd --storage-opt dm.thinp_autoextend_percent=20
+```
+
+
 ##### `dm.basesize`
 
 Specifies the size to use when creating the base device, which limits the

man/dockerd.8.md

@@ -420,6 +420,54 @@ Example use:
    $ dockerd \
          --storage-opt dm.thinpooldev=/dev/mapper/thin-pool
 
+#### dm.directlvm_device
+
+As an alternative to manually creating a thin pool as above, Docker can
+automatically configure a block device for you.
+
+Example use:
+
+   $ dockerd \
+         --storage-opt dm.directlvm_device=/dev/xvdf
+
+##### dm.thinp_percent
+
+Sets the percentage of passed in block device to use for storage.
+
+###### Example:
+
+   $ sudo dockerd \
+        --storage-opt dm.thinp_percent=95
+
+##### `dm.thinp_metapercent`
+
+Sets the percentage of the passed in block device to use for metadata storage.
+
+###### Example:
+
+   $ sudo dockerd \
+         --storage-opt dm.thinp_metapercent=1
+
+##### dm.thinp_autoextend_threshold
+
+Sets the value of the percentage of space used before `lvm` attempts to
+autoextend the available space [100 = disabled]
+
+###### Example:
+
+   $ sudo dockerd \
+         --storage-opt dm.thinp_autoextend_threshold=80
+
+##### dm.thinp_autoextend_percent
+
+Sets the value percentage value to increase the thin pool by when when `lvm`
+attempts to autoextend the available space [100 = disabled]
+
+###### Example:
+
+   $ sudo dockerd \
+         --storage-opt dm.thinp_autoextend_percent=20
+
 #### dm.basesize
 
 Specifies the size to use when creating the base device, which limits