Clarify flavor and overcommit constraints

Clarify that and instance may not overcommit itself if its flavor
requires more than the physical nodes raw (pre-overcommit) resources.

Change-Id: I65e06ff77dd2b91955bfb5ae460cc1a6ac4093fd
Closes-Bug: #1488474

doc/openstack-ops/ch_arch_compute_nodes.xml

@@ -568,6 +568,12 @@
     the instances reaches 72 GB (such as nine instances, in the case where
     each instance has 8 GB of RAM).</para>
 
+    <note>
+     <para>Regardless of the overcommit ratio, an instance can not be placed
+           on any physical node with fewer raw (pre-overcommit) resources than
+           the instance flavor requires.</para>
+    </note>
+
     <para>You must select the appropriate CPU and RAM allocation ratio for
     your particular use case.</para>
   </section>

doc/openstack-ops/ch_arch_scaling.xml

@@ -186,6 +186,12 @@
       </listitem>
     </itemizedlist>
 
+    <note>
+     <para>Regardless of the overcommit ratio, an instance can not be placed
+           on any physical node with fewer raw (pre-overcommit) resources than
+           instance flavor requires.</para>
+    </note>
+
     <para>However, you need more than the core count alone to estimate the
     load that the API services, database servers, and queue servers are likely
     to encounter. You must also consider the usage patterns of your

doc/ops-guide/source/arch_compute_nodes.rst

@@ -296,6 +296,11 @@ allocates instances to that node until the sum of the RAM associated
 with the instances reaches 72 GB (such as nine instances, in the case
 where each instance has 8 GB of RAM).
 
+.. note::
+   Regardless of the overcommit ratio, an instance can not be placed
+   on any physical node with fewer raw (pre-overcommit) resources than
+   the instance flavor requires.
+
 You must select the appropriate CPU and RAM allocation ratio for your
 particular use case.
 

doc/ops-guide/source/arch_scaling.rst

@@ -87,6 +87,11 @@ and require 80 TB of storage for ``/var/lib/nova/instances``:
 -  Default CPU overcommit ratio (``cpu_allocation_ratio`` in nova.conf)
    of 16:1.
 
+.. note::
+   Regardless of the overcommit ratio, an instance can not be placed
+   on any physical node with fewer raw (pre-overcommit) resources than
+   instance flavor requires.
+
 However, you need more than the core count alone to estimate the load
 that the API services, database servers, and queue servers are likely to
 encounter. You must also consider the usage patterns of your cloud.