Update device-mapper-driver.md

callout json-file functionality when using devicemapper - json-file logs to docker dataroot not devicemapper volumes

Author: adamancini

storage/storagedriver/device-mapper-driver.md

@@ -807,6 +807,14 @@ storage driver.
   by thin provisioning and copy-on-write. Volumes have other benefits, such as
   allowing you to share data among containers and persisting even when no
   running container is using them.
+  
+- **Note**: when using `devicemapper` and the `json-file` log driver, the log
+  files generated by a container are still stored in Docker's dataroot directory, 
+  by default `/var/lib/docker`.  If your containers generate lots of log messages, 
+  this may lead to increased disk usage or the inability to manage your system due
+  to a full disk.  You can configure a 
+  [log driver](/config/containers/logging/configure.md) to store your container
+  logs externally.
 
 ## Related Information