Document hot-replacement to resize mdadm partitions

With mdadm 3.3 or newer, partitions can be hot-replaced with new disks so that the array remains fault-tolerant through the replacement process.

xml/storage_mdadm-resize.xml

@@ -318,6 +318,39 @@
      </para>
     </step>
    </procedure>
+   <para>
+    Alternatively, if you are replacing the disks and can install the new disks
+    temporarily alongside the existing array, you can hot-replace the
+    partitions. This will keep them in service until a new partition has been
+    rebuilt as a spare, so the array does not enter a degraded state and
+    remains fault-tolerant during the process. The following steps replace
+    steps 3&ndash;5 in the above procedure.
+   </para>
+   <procedure>
+    <step>
+     <para>
+      Mark a component partition for replacement. For example, to replace
+      <filename>/dev/sda1</filename>, enter
+     </para>
+<screen>&prompt.sudo;mdadm /dev/md0 --replace /dev/sda1</screen>
+    </step>
+    <step>
+     <para>
+      Add a replacement partition to the RAID array. For example, to add
+      <filename>/dev/sdd1</filename>, enter
+     </para>
+<screen>&prompt.sudo;mdadm -a /dev/md0 /dev/sdd1</screen>
+    </step>
+    <step>
+     <para>
+      Once the new partition has been added and has finished rebuilding, the
+      partition marked for replacement will be automatically marked as faulty,
+      and can be removed from the array. For example, to remove
+      <filename>/dev/sda1</filename>, enter
+     </para>
+<screen>&prompt.sudo;mdadm /dev/md0 --remove /dev/sda1</screen>
+    </step>
+   </procedure>
   </sect2>
 
   <sect2 xml:id="sec-raid-resize-incr-raid">