doc: Added a new "Data Placement" overview section for added continuity.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>

diff --git a/doc/cluster-ops/data-placement.rst b/doc/cluster-ops/data-placement.rst
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+=========================
+ Data Placement Overview
+=========================
+
+Ceph stores, replicates and rebalances data objects across a RADOS cluster
+dynamically.  With many different users storing objects in different pools for
+different purposes on countless OSDs, Ceph operations require some data
+placement planning.  The main data placement planning concepts in Ceph include: 
+
+- **Pools:** Ceph stores data within pools, which are logical groups for storing
+  objects. Pools manage the number of placement groups, the number of replicas,
+  and the ruleset for the pool. To store data in a pool, you must have
+  an authenticated user with permissions for the pool. Ceph can snapshot pools. 
+  Future versions of Ceph will support namespaces within pools.
+  
+- **Placement Groups:** Ceph maps objects to placement groups (PGs). 
+  Placement groups (PGs) are shards or fragments of a logical object pool
+  that place objects as a group into OSDs. Placement groups reduce the amount 
+  of per-object metadata when Ceph stores the data in OSDs. A larger number of 
+  placement groups (e.g., 100 per OSD) leads to better balancing.
+
+- **CRUSH Maps:**  CRUSH is a big part of what allows Ceph to scale without 
+  performance bottlenecks, without limitations to scalability, and without a 
+  single point of failure. CRUSH maps provide the physical topology of the 
+  cluster to the CRUSH algorithm to determine where the data for an object 
+  and its replicas should be stored, and how to do so across failure domains 
+  for added  data safety among other things.
+  
+When you initially set up a test cluster, you can use the default values. Once
+you begin planning for a large Ceph cluster, refer to pools, placement groups
+and CRUSH for data placement operations. If you find some aspects challenging,
+`Inktank`_ provides excellent  premium support for Ceph.
+
+.. _Inktank: http://www.inktank.com
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