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https://issues.apache.org/jira/browse/HDFS-14978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16994240#comment-16994240
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Ayush Saxena commented on HDFS-14978:
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Thanx [~weichiu] for the design, couple of doubts :
* Does the swapBlockList() Takes care of storage quota validations?
* How is storage Policies handled here? do you retain it, There are some
policies not supported for EC, like ONE_SSD.
* What would be response, say you want to switch to 6-3 and you are able fetch
only 7 blocks, unable to write two parities, do we allow moving further,
despite moving into chances of making data more vulnerable? or do we fail?
* The Overhead of an extra tmp file still says?
> 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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