Update file system cache and multiple webnodes

Author: stevjohnson

guides/v2.0/config-guide/redis/config-redis.md

@@ -23,6 +23,11 @@ This topic discusses how to configure Redis to serve as the following Magento ca
 *	The page cache (that is, full page cache). 
 *	The default cache, which stores: configuration, layout configuration, block HTML output, collections data, reflection data, database DDL operations, EAV types and attributes, translations, web services configuration, and integration API.
 
+<div class="bs-callout bs-callout-info" id="info">
+   <span class="glyphicon-class">
+   <p>You <em>must</em> use Redis for caching if you have multiple Magento webnodes. The file system cache is inefficient and can result in errors.</p></span>
+</div>
+
 
 ### Issues with `Zend_Cache_Backend_File`
 * The `core_cache_tag` table constantly grows. If a Magento instance has multiple web sites and web stores with large catalogs, the table can grow to 15 million records in less than a day. Insertion into `core_cache_tag` leads to issues with MySQL server, including performance degradation. 

guides/v2.0/install-gde/install/post-install-config.md

@@ -16,6 +16,7 @@ Now that you've finished installing the Magento application, you need to configu
 *	<a href="#post-install-cron">Set up cron</a>
 *	<a href="#post-install-secy">Security settings</a>
 *	<a href="#post-install-rewrites">Enable Apache server rewrites</a>
+*	[Caching in a multi-webnode environment](#config-redis)
 *	<a href="#post-install-server">Server settings</a>
 *	<a href="#post-install-ee">Settings for Magento Enterprise Edition (EE) only</a>
 
@@ -40,6 +41,11 @@ If you use the Apache web server, you must enable server rewrites for pages to d
 
 <a href="{{ site.gdeurl }}install-gde/prereq/apache.html#apache-help-rewrite">Section on Apache server rewrites</a>
 
+## Caching in a multi-webnode environment {#config-redis}
+If you have multiple webnodes, you *cannot* use Magento's default file caching because there is no synchronization between webnodes. In other words, activity on one webnode is written to that webnode's file system only. Subsequent activity, if performed on another webnode, can result in unnecessary files being written or can result in errors.
+
+Instead, use [Redis]({{ site.gdeurl }}config-guide/redis/config-redis.html) for both the default cache and the page cache.
+
 <h2 id="post-install-server">Server settings</h2>
 This section briefly discusses settings we recommend you consider for the server on which Magento runs. Some of these settings are not directly related to Magento; these are provided as suggestions only.