Update instance modify documentation

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

doc/admin.rst

@@ -512,6 +512,33 @@ command::
 
 Note that this will fail if the disks already exists.
 
+Conversion of an instance's disk type
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to convert between a non-redundant instance of type
+``plain`` (LVM storage) and redundant ``drbd`` via the ``gnt-instance
+modify`` command::
+
+  # start with a non-redundant instance
+  gnt-instance add -t plain ... INSTANCE
+
+  # later convert it to redundant
+  gnt-instance stop INSTANCE
+  gnt-instance modify -t drbd INSTANCE
+  gnt-instance start INSTANCE
+
+  # and convert it back
+  gnt-instance stop INSTANCE
+  gnt-instance modify -t plain INSTANCE
+  gnt-instance start INSTANCE
+
+The conversion must be done while the instance is stopped, and
+converting from plain to drbd template presents a small risk, especially
+if the instance has multiple disks and/or if one node fails during the
+conversion procedure). As such, it's recommended (as always) to make
+sure that downtime for manual recovery is acceptable and that the
+instance has up-to-date backups.
+
 Debugging instances
 +++++++++++++++++++
 

man/gnt-instance.sgml

@@ -1397,6 +1397,13 @@ instance5: 11225
             <arg>--disk <replaceable>N</replaceable>:mode=<replaceable>MODE</replaceable></arg>
           </group>
 
+          <sbr>
+          <arg>-t<group choice="req">
+              <arg>plain</arg>
+              <arg>drbd</arg>
+            </group></arg>
+
+
           <sbr>
           <arg>--submit</arg>
           <sbr>
@@ -1416,6 +1423,13 @@ instance5: 11225
           in the form of <userinput>name=value[,...]</userinput>. For details which options can be specified, see the <command>add</command> command.
         </para>
 
+        <para>
+          The <option>-t</option> option will change the disk template
+          of the instance. Currently only conversions between the
+          plain and drbd disk templates are supported, and the
+          instance must be stopped before attempting the conversion.
+        </para>
+
         <para>
           The <option>--disk
           add:size=<replaceable>SIZE</replaceable></option> option