docs: add documentation for dm.libdm_log_level

This is a new option added specifically to allow for debugging of bugs
in Docker's storage drivers or libdm itself.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

docs/reference/commandline/dockerd.md

@@ -700,6 +700,34 @@ ENOSPC and will shutdown filesystem.
 $ sudo dockerd --storage-opt dm.xfs_nospace_max_retries=0
 ```
 
+##### `dm.libdm_log_level`
+
+Specifies the maxmimum `libdm` log level that will be forwarded to the
+`dockerd` log (as specified by `--log-level`). This option is primarily
+intended for debugging problems involving `libdm`. Using values other than the
+defaults may cause false-positive warnings to be logged.
+
+Values specified must fall within the range of valid `libdm` log levels. At the
+time of writing, the following is the list of `libdm` log levels as well as
+their corresponding levels when output by `dockerd`.
+
+| `libdm` Level | Value | `--log-level` |
+| ------------- | -----:| ------------- |
+| `_LOG_FATAL`  |     2 | error         |
+| `_LOG_ERR`    |     3 | error         |
+| `_LOG_WARN`   |     4 | warn          |
+| `_LOG_NOTICE` |     5 | info          |
+| `_LOG_INFO`   |     6 | info          |
+| `_LOG_DEBUG`  |     7 | debug         |
+
+###### Example
+
+```bash
+$ sudo dockerd \
+      --log-level debug \
+      --storage-opt dm.libdm_log_level=7
+```
+
 #### ZFS options
 
 ##### `zfs.fsname`

man/dockerd.8.md

@@ -703,6 +703,31 @@ Example use:
 
     $ sudo dockerd --storage-opt dm.xfs_nospace_max_retries=0
 
+##### dm.libdm_log_level
+
+Specifies the maxmimum libdm log level that will be forwarded to the dockerd
+log (as specified by --log-level). This option is primarily intended for
+debugging problems involving libdm. Using values other than the defaults may
+cause false-positive warnings to be logged.
+
+Values specified must fall within the range of valid libdm log levels. At the
+time of writing, the following is the list of libdm log levels as well as their
+corresponding levels when output by dockerd.
+
+| libdm Level | Value | --log-level |
+| ----------- | -----:| ----------- |
+| _LOG_FATAL  |     2 | error       |
+| _LOG_ERR    |     3 | error       |
+| _LOG_WARN   |     4 | warn        |
+| _LOG_NOTICE |     5 | info        |
+| _LOG_INFO   |     6 | info        |
+| _LOG_DEBUG  |     7 | debug       |
+
+Example use:
+
+    $ sudo dockerd \
+	      --log-level debug \
+          --storage-opt dm.libdm_log_level=7
 
 ## ZFS options