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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3370) HDFS hardlink
Date Tue, 05 Jun 2012 20:24:23 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13289689#comment-13289689
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Lars Hofhansl commented on HDFS-3370:
-------------------------------------

Reading through the Design Doc it seems that FileSystem.{setPermission|setOwner} would be
awkward. We'd have to find each INodeHardLinkFile pointing to the same "file" and then changing
all their permissions/owners.

HardLinkFileInfo could also maintain permissions and owners (since they - following posix
- are the same for each hard link). That way changing owner or permissions would immediately
affect all hard links.
When the fsimage is saved each INodeHardLinkFile would still write its own permission and
owner (for simplicity, but that could be optimized, as long as at least one INode writes the
permissions/owner).
Upon read INode representing a hardlink must have the same permission/owner as all other INodes
linking to the same "file". If not the image is inconsistent.

In that case HardLinkFileInfo would not need to maintain a list of pointers back to all INodeHardLinkFiles,
and owner/permissions would only be stored once in memory.

                
> 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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