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From "M. C. Srivas (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3154) Add a notion of immutable/mutable files
Date Wed, 28 Mar 2012 07:08:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3154?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13240244#comment-13240244
] 

M. C. Srivas commented on HDFS-3154:
------------------------------------

Ok, so the Andrew File System, from which Ceph draws a lot of inspiration, had extensive client-side
caching, and snapshots and mirrors. Plus the files were 100% mutable with stronger consistency
semantics than NFS.  And unlike Ceph and more like HDFS, AFS ran on the local file system
that came bundled with the operating system.

So it is not hard to implement. The source code for AFS is out there in public domain. 

Making every file immutable introduces more complications for the users of HDFS. It may simplify
some things for the engineers developing it. But the users have to deal with it on a daily
basis. I'd rather that the engineers solve the hard problems and provide simplicity to their
users.

I suspect underlying all this is a use-case that is in the back of people's minds, for which
one solution might involve making every file permanently immutable. It would be beneficial
to illustrate the use-case and discuss.
                
> 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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