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| <h1>Datanodes</h1> |
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| <!--- |
| Licensed to the Apache Software Foundation (ASF) under one or more |
| contributor license agreements. See the NOTICE file distributed with |
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| The ASF licenses this file to You under the Apache License, Version 2.0 |
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| See the License for the specific language governing permissions and |
| limitations under the License. |
| --> |
| <p>Datanodes are the worker bees of Ozone. All data is stored on data nodes. |
| Clients write data in terms of blocks. Datanode aggregates these blocks into |
| a storage container. A storage container is the data streams and metadata |
| about the blocks written by the clients.</p> |
| <h2 id="storage-containers">Storage Containers</h2> |
| <p><img src="ContainerMetadata.png" alt="FunctionalOzone"></p> |
| <p>A storage container is a self-contained super block. It has a list of Ozone |
| blocks that reside inside it, as well as on-disk files which contain the |
| actual data streams. This is the default Storage container format. From |
| Ozone’s perspective, container is a protocol spec, actual storage layouts |
| does not matter. In other words, it is trivial to extend or bring new |
| container layouts. Hence this should be treated as a reference implementation |
| of containers under Ozone.</p> |
| <h2 id="understanding-ozone-blocks-and-containers">Understanding Ozone Blocks and Containers</h2> |
| <p>When a client wants to read a key from Ozone, the client sends the name of |
| the key to the Ozone Manager. Ozone manager returns the list of Ozone blocks |
| that make up that key.</p> |
| <p>An Ozone block contains the container ID and a local ID. The figure below |
| shows the logical layout out of Ozone block.</p> |
| <p><img src="OzoneBlock.png" alt="OzoneBlock"></p> |
| <p>The container ID lets the clients discover the location of the container. The |
| authoritative information about where a container is located is with the |
| Storage Container Manager (SCM). In most cases, the container location will be |
| cached by Ozone Manager and will be returned along with the Ozone blocks.</p> |
| <p>Once the client is able to locate the container, that is, understand which |
| data nodes contain this container, the client will connect to the datanode |
| and read the data stream specified by <em>Container ID:Local ID</em>. In other |
| words, the local ID serves as index into the container which describes what |
| data stream we want to read from.</p> |
| <h3 id="discovering-the-container-locations">Discovering the Container Locations</h3> |
| <p>How does SCM know where the containers are located ? This is very similar to |
| what HDFS does; the data nodes regularly send container reports like block |
| reports. Container reports are far more concise than block reports. For |
| example, an Ozone deployment with a 196 TB data node will have around 40 |
| thousand containers. Compare that with HDFS block count of million and half |
| blocks that get reported. That is a 40x reduction in the block reports.</p> |
| <p>This extra indirection helps tremendously with scaling Ozone. SCM has far |
| less block data to process and the namespace service (Ozone Manager) as a |
| different service are critical to scaling Ozone.</p> |
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