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From "Weihua Jiang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-7285) Erasure Coding Support inside HDFS
Date Fri, 24 Oct 2014 13:03:33 GMT
Weihua Jiang created HDFS-7285:

             Summary: Erasure Coding Support inside HDFS
                 Key: HDFS-7285
                 URL: https://issues.apache.org/jira/browse/HDFS-7285
             Project: Hadoop HDFS
          Issue Type: New Feature
            Reporter: Weihua Jiang
            Assignee: Zhe Zhang

Erasure Coding (EC) can greatly reduce the storage overhead without sacrifice of data reliability,
comparing to the existing HDFS 3-replica approach. For example, if we use a 10+4 Reed Solomon
coding, we can allow loss of 4 blocks, with storage overhead only being 40%. This makes EC
a quite attractive alternative for big data storage, particularly for cold data. 

Facebook had a related open source project called HDFS-RAID. It used to be one of the contribute
packages in HDFS but had been removed since Hadoop 2.0 for maintain reason. The drawbacks
are: 1) it is on top of HDFS and depends on MapReduce to do encoding and decoding tasks; 2)
it can only be used for cold files that are intended not to be appended anymore; 3) the pure
Java EC coding implementation is extremely slow in practical use. Due to these, it might not
be a good idea to just bring HDFS-RAID back.

We (Intel and Cloudera) are working on a design to build EC into HDFS that gets rid of any
external dependencies, makes it self-contained and independently maintained. This design lays
the EC feature on the storage type support and considers compatible with existing HDFS features
like caching, snapshot, encryption, high availability and etc. This design will also support
different EC coding schemes, implementations and policies for different deployment scenarios.
By utilizing advanced libraries (e.g. Intel ISA-L library), an implementation can greatly
improve the performance of EC encoding/decoding and makes the EC solution even more attractive.
We will post the design document soon. 

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