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From "M. C. Srivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3370) HDFS hardlink
Date Wed, 16 May 2012 04:56:10 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13276461#comment-13276461
] 

M. C. Srivas commented on HDFS-3370:
------------------------------------

Sanjay,  POSIX says that a user cannot open a file unless they have permissions to traverse
the entire path from / to the file.  The problem is that if a file has two paths (as in a
hard-link), perms becomes very hard to enforce since a file does not know which dir is its
parent. Imagine a rename of a file with many hard-links across to a new dir. This problem
is harder in a distr file system if you wish to spread the meta-data.  Note that the enforcement
happens automatically with symbolic links. As you point out, with MapR we _could_ implement
hard-links within a volume, but chose not to and instead implemented only symlinks.  (I personally
find symlinks to be more flexible). 

                
> 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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