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From "M. C. Srivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3370) HDFS hardlink
Date Wed, 09 May 2012 14:33:48 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271450#comment-13271450

M. C. Srivas commented on HDFS-3370:

Creating hard-links in a distributed file-system will cause all kinds of future problems with
scalability. Hard-links are rarely used in the real-world, because of all the associated bizzare
problems.  Eg,  consider a hardlink setup as follows:

link1:   /path1/dirA/file
link2:   /path2/dirB/file

1. Changing the permissions along the path "/path1/dirA"  to make "file" inaccessible works,
but now with hard-links "/path2/dirB" is wide open.

2. Rename "/path2/dirB" to "/path3/dirC"  will require taking locks on "/path1/dirA" ... but
the "file" does not have "parent ptrs" to figure out which path(s) to lock.

I would recommend finding a different approach to implementing snapshots than adding this
> 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
> 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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