Add info about __DEFAULT__ in install docs
This patch adds info regarding the __DEFAULT__ type. Also how
it can be removed and who should remove it.
@@ -34,3 +34,43 @@ cinder-backup daemon
Messaging queue
Routes information between the Block Storage processes.
+
+The default volume type
+-----------------------
+
+Since the Train release, it is required that each volume must have a
+*volume type*, and thus the required configuration option
+``default_volume_type`` must have a value. A system-defined volume type
+named ``__DEFAULT__`` is created in the database during installation and
+is the default value of the ``default_volume_type`` configuration option.
+
+You (or your deployment tool) may wish to have a different volume type that
+is more suitable for your particular installation as the default type.
+This can be accomplished by creating the volume type you want using the
+Block Storage API, and then setting that volume type as the value for
+the configuration option. (The latter operation, of course, cannot be
+done via the Block Storage API.)
+
+The system defined ``__DEFAULT__`` volume type is a regular volume type
+that may be updated or deleted. There is nothing special about it. It only
+exists because there must always be at least one volume type in a cinder
+deployment, and before the Block Storage API comes up, there is no way for
+there to be a volume type unless the system creates it.
+
+Given that since the Victoria release it is possible to set a default
+volume type for any project, having a volume type named ``__DEFAULT__``
+in your deployment may be confusing to your users, leading them to think this
+is the type that will be assigned while creating volumes (if the user doesn't
+specify one) or them specifically requesting ``__DEFAULT__`` when creating a
+volume instead of the actual configured default type for the system or their
+project.
+
+If you don't wish to use the ``__DEFAULT__`` type, you may delete it. The
+Block Storage API will prevent deletion under these circumstances:
+
+* If ``__DEFAULT__`` is the value of the ``default_volume_type`` configuration
+ option then it cannot be deleted. The solution is to make a different
+ volume type the value of that configuration option.
+* If there are volumes in the deployment of the ``__DEFAULT__`` type, then
+ it cannot be deleted. The solution is to retype those volumes to some
+ other appropriate volume type.
