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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 Thu, 29 Mar 2012 02:45:27 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3154?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13240936#comment-13240936
] 

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

> So it is not hard to implement. ...

I am saying caching on immutable files is more efficient in run time performance but not about
the difficulty of implementation.

> Making every file immutable introduces more complications for the users of HDFS. ...

We are not making all files immutable.  Users could create mutable files.

But I do propose that the default is to file creation is immutable since all the current applications
requires only immutable (since append is not a stable release yet.)  It opens an opportunity
for performance improvement.  For example, [Scott's comment|http://mail-archives.apache.org/mod_mbox/hadoop-hdfs-dev/201203.mbox/%3CCB927556.8DE2B%25scott@richrelevance.com%3E]
on extent can be implemented only for mutable files.
                
> 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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