docs/architecture: update volume design for raw block device

Refs: #2421
Signed-off-by: Sylvain Laperche <[email protected]>

docs/developer/architecture/volume.rst

@@ -34,6 +34,7 @@ Goals
 * add support for volume deletion (one by one) in the Platform UI
 * add support for volume listing/monitoring (show status, size, …) in the
   Platform UI
+* expose raw block device (unformated) as **Volume**
 * document how to create a volume
 * document how to create a **StorageClass** object
 * automated tests on volume workflow (creation, deletion, …)
@@ -44,7 +45,6 @@ Non-Goals
 
 * RAID support
 * LVM support
-* expose raw block device (unformated) as **Volume**
 * use an **Admission Controller** for semantic validation
 * auto-discovery of the disks
 * batch provisioning from the Platform UI
@@ -148,6 +148,21 @@ Similarly, our **Volume** object will have the following states:
 * **Terminating**: cleanup of the backing storage in progress (e.g.
   an asynchronous Salt call is still running).
 
+Storage Class
+^^^^^^^^^^^^^
+
+Storage classes are used to specify how to format (if necessary) the backing
+device (this is handled by Salt).
+
+The supported ``fsType`` are:
+
+* **ext4**
+* **xfs**
+* **none**: for raw block device, without any filesystem (not supported for
+  **SparseLoopDevice**).
+
+Formating options can be specified in `mkfsOptions`, as a JSON-encoded list of
+parameters to pass to ``mkfs``.
 
 Operator Reconciliation Loop
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^