Adds a storage concepts explanation

fixes bug 938365

There was a lack of storage concepts in the early parts of the
documentation. This adds a new section with a kick-start table
to allow first time readers to quickly understand the different
types of storage in an OpenStack installation.

Just a few edits -- double-check the ephemeral explanation as
I took it from the aboutcompute.xml file.

Change-Id: I447423f98bc3e36b6018179cdd81d8bb6d56b221

doc/src/docbkx/common/getstart.xml

@@ -616,4 +616,5 @@ xml:id="ch_getting-started-with-openstack">
       <para>Like Quantum, Cinder will mainly interact with Nova, providing volumes for its instances.</para>
     </section>
   </section>  
+  <xi:include href="storage-concepts.xml"/>
 </chapter>

doc/src/docbkx/common/storage-concepts.xml

@@ -0,0 +1,79 @@
+<?xml version="1.0" encoding="utf-8"?>
+<section xmlns="http://docbook.org/ns/docbook"
+xmlns:xi="http://www.w3.org/2001/XInclude"
+xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
+xml:id="storage-concepts">
+  <title>Storage Concepts</title>
+  <para>
+  Storage is found in many parts of the OpenStack stack, and the
+  differing types can cause confusion to even experienced cloud
+  engineers. Here's a simple chart to kick-start your understanding:
+  </para>
+  <table rules="all">
+    <caption>Types of Storage</caption>
+    <col width="33%" />
+    <col width="33%" />
+    <col width="33%" />
+    <thead>
+      <tr>
+        <td>On-instance / ephemeral </td>
+        <td>Volumes block storage (Cinder)</td>
+        <td>Object Storage (Swift)</td>
+      </tr>
+    </thead>
+    <tbody>
+       <tr>
+        <td>Used for running Operating System and scratch space</td>
+        <td>Used for adding additional persistent storage to a virtual
+          machine (VM)</td>
+        <td>Used for storing virtual machine images and data</td>
+      </tr>
+     <tr>
+        <td>Persists until VM is terminated</td>
+        <td>Persists until deleted</td>
+        <td>Persists until deleted</td>
+      </tr>
+     <tr>
+        <td>Access associated with a VM</td>
+        <td>Access associated with a VM</td>
+        <td>Available from anywhere</td>
+      </tr>
+     <tr>
+        <td>Implemented as a filesystem underlying OpenStack Compute</td>
+        <td>Mounted via OpenStack Block-Storage controlled protocol
+          (for example, iSCSI)</td>
+        <td>REST API</td>
+      </tr>
+     <tr>
+        <td>Administrator configures size setting, based on
+          flavors</td>
+        <td>Sizings based on need</td>
+        <td>Easily scalable for future growth</td>
+      </tr>
+     <tr>
+        <td>Example: 10GB first disk, 30GB/core second disk</td>
+        <td>Example: 1TB "extra hard drive"</td>
+        <td>Example: 10s of TBs of dataset storage</td>
+      </tr>
+    </tbody>
+  </table>
+ <para> Other points of note include:
+ <itemizedlist>
+ <listitem><para><emphasis>OpenStack Object Storage is not used like a
+            traditional hard drive.</emphasis> Object storage is all
+          about relaxing some of the constraints of a POSIX-style file
+          system. The access to it is API-based (and the API uses
+          http). This is a good idea as if you don't have to provide
+          atomic operations (that is, you can rely on eventual
+          consistency), you can much more easily scale a storage
+          system and avoid a central point of failure. </para></listitem>
+  <listitem><para><emphasis>The OpenStack Image Service is used to manage
+  the virtual machine images in an OpenStack cluster, not store them.
+  </emphasis> Instead, it provides an abstraction to different methods
+  for storage - a bridge to the storage, not the storage itself.</para></listitem>
+  <listitem><para><emphasis>OpenStack Object Storage can function on its
+            own.</emphasis> The Object Storage (swift) product can be
+          used independently of the Compute (nova) product.</para></listitem>
+ </itemizedlist>
+ </para>
+</section>