Document memcache max size with shard ranges

When you start getting more then 3k shards in a root container the
cached shard range listing can get bigger then the default max size for
memcache (1MB).

So add a mention about it in the configuration guide.

Fixes bug 1890643

Change-Id: If380410c17ed9ebc014b8198af0ea8d502deacc8

doc/source/deployment_guide.rst

@@ -492,6 +492,29 @@ on any servers that have available RAM and CPU.  At Rackspace, we run
 Memcached on the proxy servers.  The ``memcache_servers`` config option
 in the ``proxy-server.conf`` should contain all memcached servers.
 
+*************************
+Shard Range Listing Cache
+*************************
+
+When a container gets :ref:`sharded<sharding_doc>` the root container will still be the
+primary entry point to many container requests, as it provides the list of shards.
+To take load off the root container Swift by default caches the list of shards returned.
+
+As the number of shards for a root container grows to more than 3k the memcache default max
+size of 1MB can be reached. Care is needed to keep an eye on the current value or resize it
+to something like 5MB.
+
+Memcache has a ``stats sizes`` option that can point out the current size usage. As this
+reaches the current max an increase might be in order::
+
+  # telnet <memcache server> 11211
+  > stats sizes
+  STAT 160 2
+  STAT 448 1
+  STAT 576 1
+  END
+
+
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 System Time
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