diff --git a/README.rst b/README.rst
index dcd9462..e4488fa 100644
--- a/README.rst
+++ b/README.rst
@@ -357,6 +357,21 @@
               enabled: false
 
 
+In case some custom block devices should be used (like loop devices for testing purpose),
+it is needed to indicate proper partition prefix.
+
+.. code-block:: yaml
+
+    ceph:
+      osd:
+        backend:
+          bluestore:
+            disks:
+            - dev: /dev/loop20
+              block_db: /dev/loop21
+              data_partition_prefix: 'p'
+
+
 Ceph client roles - ...Deprecated - use ceph:common instead
 --------------------------------------------------------