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[GitHub] [ozone] umamaheswararao commented on a change in pull request #3006: HDDS-6172: EC: Document the Ozone EC
umamaheswararao commented on a change in pull request #3006:
URL: https://github.com/apache/ozone/pull/3006#discussion_r789444758
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File path: hadoop-hdds/docs/content/feature/ErasureCoding.md
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@@ -0,0 +1,215 @@
+---
+title: "Ozone Erasure Coding"
+weight: 1
+menu:
+ main:
+ parent: Features
+summary: Erasure Coding Support for Ozone.
+---
+<!---
+ Licensed to the Apache Software Foundation (ASF) under one or more
+ contributor license agreements. See the NOTICE file distributed with
+ this work for additional information regarding copyright ownership.
+ The ASF licenses this file to You 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.
+-->
+
+## Background
+
+Distributed systems basic expectation is to provide the data durability.
+To provide the higher data durability, many popular storage systems use replication
+approach which is expensive. The Apache Ozone supports `RATIS/THREE` replication scheme.
+The Ozone default replication scheme `RATIS/THREE` has 200% overhead in storage
+space and other resources (e.g., network bandwidth).
+However, for warm and cold datasets with relatively low I/O activities, additional
+block replicas rarely accessed during normal operations, but still consume the same
+amount of resources as the first replica.
+
+Therefore, a natural improvement is to use Erasure Coding (EC) in place of replication,
+which provides the same level of fault-tolerance with much less storage space.
+In typical Erasure Coding (EC) setups, the storage overhead is no more than 50%.
+The replication factor of an EC file is meaningless. Instead of replication factor,
+we introduced ReplicationConfig interface to specify the required type of replication,
+either `RATIS/THREE` or `EC`.
+
+Integrating EC with Ozone can improve storage efficiency while still providing similar
+data durability as traditional replication-based Ozone deployments.
+As an example, a 3x replicated file with 6 blocks will consume 6*3 = `18` blocks of disk space.
+But with EC (6 data, 3 parity) deployment, it will only consume `9` blocks of disk space.
+
+## Architecture
+
+The storage data layout is a key factor in the implementation of EC. After deep analysis
+and several technical consideration, the most fitting data layout is striping model.
+The data striping layout is not new. The striping model already adapted by several other
+file systems(Ex: Quantcast File System, Hadoop Distributed File System etc) successfully before.
+
+For example, with the EC (6 data, 3 parity) scheme, the data chunks will be distributed to first 6 data nodes in order
+and then client generates the 3 parity chunks and transfer to remaining 3 nodes in order.
+These 9 chunks together we call as "Stripe". Next 6 chunks will be distributed to the same first 6 data nodes again
+and the parity to remaining 3 nodes. These 9 data nodes stored blocks together called as "BlockGroup".
+
+If the application is continuing to write beyond teh size of `6 * BLOCK_SIZE`, then client will request new block group from Ozone Manager.
+
+### Erasure Coding Write
+
+The core logic of erasure coding writes are placed at ozone client.
+When client creates the file, ozone manager allocates the block group(`d + p`)
+number of nodes from the pipeline provider and return the same to client.
+As data is coming in from the application, client will write first d number of chunks
+to d number of data nodes in block group. It will also cache the d number chunks
+to generate the parity chunks. Once parity chunks generated, it will transfer the
+same to the remaining p nodes in order. Once all blocks reached their configured sizes,
+client will request the new block group nodes.
+
+Below diagram depicts the block allocation in containers as logical groups.
+For interest of space, we assumed EC(3, 2) replication config for the diagram.
+
+
+
+
+Let's zoom out the blockID: 1 data layout from the above picture, that showed in the following picture.
+This picture shows how the chunks will be layed out in data node blocks.
+
+
+Currently ec client re-used the data transfer end-points to transfer the data to data nodes.
+That is XceiverGRPC client, used for writing data and for sending putBlock info.
+Since it used the existing transfer protocols while transferring the data, design got big advantage that,
+datanode sides changes are very minimal. The data block at data nodes would be written
+same as any other block in non-ec mode. In a single block group, container id numbers
+are same in all nodes. A file can have multiple block groups. Each block group will
+have `d+p` number of block and all ids are same.
+
+**d** - Number of data blocks in a block group
+
+**p** - Number of parity blocks in a block group
+
+### Erasure Coding Read
+
+For reads, OM will provide the node location details as part of key lookup.
+If the key is in EC, Ozone client will do the reads in EC fashion. Since the data lay out
+is different(Previous section discussed the layout), reads should consider the layout and do the reads.
+
+EC client will open the connections to DNs based on the expected locations. When all locations are available,
+it will attempt to do plain reads chunk by chunk in round robin fashion from d data blocks.
+
+Below picture shows the order when there are no failures while reading.
+
+
+Until it sees read failures, there is no need of doing EC reconstruction.
+
+#### Erasure Coding On-the-fly Reconstruction Reads
+
+When client detects there are failures while reading or when starting the reads,
+Ozone EC client is capable of reconstructing/recovering the lost data by doing the ec decoding.
+To do the ec decoding it needs to read parity replicas. This is a degraded read as it needs to do reconstruction.
+This reconstruction is completely transparent to the applications.
+
+Below picture depicts how it uses parity replicas in reconstruction.
+
Review comment:
Looks like many of the ozone docs followed the above approach. I already ran mvn site and it's generated fine to me.
Is there an issue with existing way to referencing? ( This is working for both intellij and site to me)
Please check this [mvn site generated file screenshot ](https://issues.apache.org/jira/secure/attachment/13039196/mvn-site-Ozone-EC-doc-screenshot.png)
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