[jira] [Created] (HDFS-14978) In-place Erasure Coding Conversion

["Wei-Chiu Chuang (Jira)" <ji...@apache.org>]

Wei-Chiu Chuang created HDFS-14978:
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             Summary: In-place Erasure Coding Conversion
                 Key: HDFS-14978
                 URL: https://issues.apache.org/jira/browse/HDFS-14978
             Project: Hadoop HDFS
          Issue Type: New Feature
          Components: erasure-coding
    Affects Versions: 3.0.0
            Reporter: Wei-Chiu Chuang
            Assignee: Wei-Chiu Chuang


HDFS Erasure Coding is a new feature added in Apache Hadoop 3.0. It uses encoding algorithms to reduce disk space usage while retaining redundancy necessary for data recovery. It was a huge amount of work but it is just getting adopted after almost 2 years.

One usability problem that’s blocking users from adopting HDFS Erasure Coding is that existing replicated files have to be copied to an EC-enabled directory explicitly. Renaming a file/directory to an EC-enabled directory does not automatically convert the blocks. Therefore users typically perform the following steps to erasure-code existing files:


{noformat}
Create $tmp directory, set EC policy at it
Distcp $src to $tmp
Delete $src (rm -rf $src)
mv $tmp $src
{noformat}


There are several reasons why this is not popular:
* Complex. The process involves several steps: distcp data to a temporary destination; delete source file; move destination to the source path.
* Availability: there is a short period where nothing exists at the source path, and jobs may fail unexpectedly.
* Overhead. During the copy phase, there is a point in time where all of source and destination files exist at the same time, exhausting disk space.
* Not snapshot-friendly. If a snapshot is taken prior to performing the conversion, the source (replicated) files will be preserved in the cluster too. Therefore, the conversion actually increase storage space usage.
* Not management-friendly. This approach changes file inode number, modification time and access time. Erasure coded files are supposed to store cold data, but this conversion makes data “hot” again.
* Bulky. It’s either all or nothing. The directory may be partially erasure coded, but this approach simply erasure code everything again.

To ease data management, we should offer a utility tool to convert replicated files to erasure coded files in-place.



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