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Date: Thu, 22 Mar 2012 20:34:23 +0000 (UTC)
From: "Milind Bhandarkar (Commented) (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (HDFS-3107) HDFS truncate
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    [ https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13235996#comment-13235996 ] 

Milind Bhandarkar commented on HDFS-3107:
-----------------------------------------

Thought a little more about this. The namenode RPC should have length as exactly a multiple of blocksize, otherwise it should throw exception. (so, option 1 in my comment above.) And concatFile could be null, so that case where files need to be truncated to exactly block boundary (option 1 in Nicholas's comment) can be trivially supported.
                
> HDFS truncate
> -------------
>
>                 Key: HDFS-3107
>                 URL: https://issues.apache.org/jira/browse/HDFS-3107
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: data-node, name-node
>            Reporter: Lei Chang
>         Attachments: HDFS_truncate_semantics_Mar15.pdf, HDFS_truncate_semantics_Mar21.pdf
>
>   Original Estimate: 1,344h
>  Remaining Estimate: 1,344h
>
> Systems with transaction support often need to undo changes made to the underlying storage when a transaction is aborted. Currently HDFS does not support truncate (a standard Posix operation) which is a reverse operation of append, which makes upper layer applications use ugly workarounds (such as keeping track of the discarded byte range per file in a separate metadata store, and periodically running a vacuum process to rewrite compacted files) to overcome this limitation of HDFS.

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