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Date: Mon, 26 Mar 2012 21:14:27 +0000 (UTC)
From: "Scott Carey (Commented) (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-3154) Add a notion of immutable/mutable
 files
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    [ https://issues.apache.org/jira/browse/HDFS-3154?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13238841#comment-13238841 ] 

Scott Carey commented on HDFS-3154:
-----------------------------------

You should be able to make a copy of an immutable file that takes up no extra space, but can be appended to or truncated.  The immutable file and the copy would share immutable blocks.
                
> Add a notion of immutable/mutable files
> ---------------------------------------
>
>                 Key: HDFS-3154
>                 URL: https://issues.apache.org/jira/browse/HDFS-3154
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: name-node
>            Reporter: Tsz Wo (Nicholas), SZE
>            Assignee: Tsz Wo (Nicholas), SZE
>
> The notion of immutable file is useful since it lets the system and tools optimize certain things as discussed in [this email thread|http://mail-archives.apache.org/mod_mbox/hadoop-hdfs-dev/201203.mbox/%3CCAPn_vTuZomPmBTypP8_1xTr49Sj0fy7Mjhik4DbcAA+BLH53=g@mail.gmail.com%3E].  Also, many applications require only immutable files.  Here is a proposal:
> - Immutable files means that the file content is immutable.  Operations such as append and truncate that change the file content are not allowed to act on immutable files.  However, the meta data such as replication and permission of an immutable file can be updated.  Immutable files can also be deleted or renamed.
> - Users have to pass immutable/mutable as a flag in file creation.  This is an unmodifiable property of the created file.
> - If users want to change the data in an immutable file, the file could be copied to another file which is created as mutable.

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