document system_disk attribute

admin-guide/deployment/partitioning.rst

@@ -4,8 +4,14 @@ Partitioning
 Partitions system are configured with a standard schema using LVM, so
 that they can be possibly changed afterwards.
 
-By default the whole space available on the  first block device is used
+By default the whole space available on the first block device is used
 and any existing partition is removed.
+
+Alternatively you might set the sysdb attribute ``system_disk`` with the
+device name of the disk which should be used instead::
+
+    bob node set-attr $FQDN system_disk=md126
+
 The default partition schema is:
 
    - create one primary ``/boot`` partition of 1Gb;