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https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13293782#comment-13293782
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Liyin Tang commented on HDFS-3370:
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Good point, Lars.
When users run "cp" in the linux file system against hard linked files, it will 
copy the bytes, right?
I think we shall keep the same semantics here as well. 

In terms of optimization, the upper level application shall have the knowledge 
when to use hardlink in the remote/destination DFS instead of coping the bytes 
between two clusters.
                
> HDFS hardlink
> -------------
>
>                 Key: HDFS-3370
>                 URL: https://issues.apache.org/jira/browse/HDFS-3370
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Hairong Kuang
>            Assignee: Liyin Tang
>         Attachments: HDFS-HardLink.pdf
>
>
> We'd like to add a new feature hardlink to HDFS that allows harlinked files 
> to share data without copying. Currently we will support hardlinking only 
> closed files, but it could be extended to unclosed files as well.
> Among many potential use cases of the feature, the following two are 
> primarily used in facebook:
> 1. This provides a lightweight way for applications like hbase to create a 
> snapshot;
> 2. This also allows an application like Hive to move a table to a different 
> directory without breaking current running hive queries.

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