HDFS-12551. Ozone: Documentation: Add Ozone overview documentation. Contributed by Anu Engineer.

hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/OzoneOverview.md

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+<!---
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License. See accompanying LICENSE file.
+-->
+Ozone Overview
+==============
+
+
Ozone is an Object store for Apache Hadoop. It aims to scale to billions of
+keys. 
The following is a high-level overview of the core components of Ozone.


+
+![Ozone Architecture Overview](images/ozoneoverview.png) 


+
+The main elements of Ozone are
:
+
+### Clients
+Ozone ships with a set of ready-made clients. They are 
Ozone CLI and Corona.

+
+    * [Ozone CLI](./OzoneCommandShell.html) is the command line interface like 'hdfs' command.

+
+    * Corona is a  load generation tool for Ozone.

+
+### REST Handler
+Ozone provides both an RPC (Remote Procedure Call) as well as a  REST
+(Representational State Transfer) style interface. This allows clients to be
+written in many languages quickly. Ozone strives to maintain a similar
+interface between REST and RPC. The Rest handler offers the REST protocol
+services of Ozone.
+
+For most purposes, a client can make one line change to switch from REST to
+RPC or vice versa.  

+
+### Ozone File System
+Ozone file system (TODO: Add documentation) is a Hadoop compatible file system.
+This is the important user-visible component of ozone.
+This allows Hadoop services and applications like Hive/Spark to run against
+Ozone without any change.
+
+### Ozone Client
+This is like DFSClient in HDFS. This acts as the standard client to talk to
+Ozone. All other components that we have discussed so far rely on Ozone client
+(TODO: Add Ozone client documentation).

+
+### Key Space Manager

+Key Space Manager(KSM) takes care of the Ozone's namespace.
+All ozone entities like volumes, buckets and keys are managed by KSM
+(TODO: Add KSM documentation). In Short, KSM is the metadata manager for Ozone.
+KSM talks to blockManager(SCM) to get blocks and passes it on to the Ozone
+client.  Ozone client writes data to these blocks.
+KSM will eventually be replicated via Apache Ratis for High Availability.

+
+### Storage Container Manager
+Storage Container Manager (SCM) is the block and cluster manager for Ozone.
+SCM along with data nodes offer a service called 'containers'.
+A container is a group unrelated of blocks that are managed together
+as a single entity.
+
+SCM offers the following abstractions.


+
+![SCM Abstractions](images/scmservices.png)
+#### Blocks
+Blocks are like blocks in HDFS. They are replicated store of data.
+
+#### Containers
+A collection of blocks replicated and managed together.
+
+#### Pipelines
+SCM allows each container to choose its method of replication.
+For example, a container might decide that it needs only one copy of a  block
+and might choose a stand-alone pipeline. Another container might want to have
+a very high level of reliability and pick a RATIS based pipeline. In other
+words, SCM allows different kinds of replication strategies to co-exist.
+
+#### Pools
+A group of data nodes is called a pool. For scaling purposes,
+we define a pool as a set of machines. This makes management of datanodes
+easier.
+
+#### Nodes
+The data node where data is stored.

hadoop-project/src/site/site.xml

@@ -111,6 +111,8 @@
 
     <menu name="Ozone" inherit="top">
       <item name="Getting Started" href="hadoop-project-dist/hadoop-hdfs/OzoneGettingStarted.html"/>
+      <item name="Ozone Overview"
+            href="hadoop-project-dist/hadoop-hdfs/OzoneOverview.html"/>
       <item name="Commands Reference" href="hadoop-project-dist/hadoop-hdfs/OzoneCommandShell.html"/>
       <item name="Ozone Rest API" href="hadoop-project-dist/hadoop-hdfs/OzoneRest.html"/>
       <item name="Ozone Metrics" href="hadoop-project-dist/hadoop-hdfs/OzoneMetrics.html"/>