planning-architecture-input_model-concepts-servergroups_failurezones.xml

<?xml version="1.0"?>
<!DOCTYPE section [
 <!ENTITY % entities SYSTEM "entity-decl.ent"> %entities;
]>
<section xmlns="http://docbook.org/ns/docbook" xmlns:xi="http://www.w3.org/2001/XInclude"
    xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="concept-servergroups-failurezones"
    version="5.1">
 <title>Server Groups and Failure Zones</title>
 <para>
  A <guimenu>control-plane</guimenu> defines a list of
  <guimenu>server-groups</guimenu> as the failure zones from which it wants to
  use servers. All servers in a <guimenu>server-group</guimenu> listed as a
  failure zone in the <guimenu>control-plane</guimenu> and any
  <guimenu>server-groups</guimenu> they contain are considered part of that
  failure zone for allocation purposes. The following example shows how three
  levels of <guimenu>server-groups</guimenu> can be used to model a failure
  zone consisting of multiple racks, each of which in turn contains a number of
  <guimenu>servers</guimenu>.
 </para>
 <informalfigure>
  <mediaobject>
   <imageobject role="fo">
    <imagedata fileref="media-inputmodel-hphelionopenstack_servergroups.png" width="75%"/>
   </imageobject>
   <imageobject role="html">
    <imagedata fileref="media-inputmodel-hphelionopenstack_servergroups.png"/>
   </imageobject>
  </mediaobject>
 </informalfigure>
 <!-- FIXME: <para>
  <link xlink:href="../../../../media/inputmodel/hphelionopenstack_servergroups_lg.png">Download
  a high-res version</link>
 </para> -->
 <para>
  When allocating <guimenu>servers</guimenu>, the configuration processor will
  traverse down the hierarchy of <guimenu>server-groups</guimenu> listed as
  failure zones until it can find an available server with the required
  <guimenu>server-role</guimenu>. If the allocation policy is defined to be
  strict, it will allocate <guimenu>servers</guimenu> equally across each of
  the failure zones. A <guimenu>cluster</guimenu> or
  <guimenu>resource-group</guimenu> can also independently specify the failure
  zones it wants to use if needed.
 </para>
</section>