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From "Tsz Wo (Nicholas), SZE (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3154) Add a notion of immutable/mutable files
Date Tue, 27 Mar 2012 02:41:25 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3154?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13239126#comment-13239126
] 

Tsz Wo (Nicholas), SZE commented on HDFS-3154:
----------------------------------------------

> Can you list the concrete benefits you think would come from having some, but not all,
files be immutable? ...

The main benefit is caching.

Another benefit is to protect the files.  It avoid accidentally append/truncate on immutable
files.  I did not know this was common until recently [from Milind|https://issues.apache.org/jira/browse/HDFS-3107?focusedCommentId=13233609&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13233609].
                
> 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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