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

["Shilun Fan (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-14978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17802423#comment-17802423 ] 

Shilun Fan edited comment on HDFS-14978 at 1/4/24 8:14 AM:
-----------------------------------------------------------

Bulk update: moved all 3.4.0 non-blocker issues, please move back if it is a blocker. Retarget 3.5.0.


was (Author: slfan1989):
updated the target version for preparing 3.4.0 release.

> 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: Aravindan Vijayan
>            Priority: Major
>         Attachments: In-place Erasure Coding Conversion.pdf
>
>
> 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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