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From "Milind Bhandarkar (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3107) HDFS truncate
Date Tue, 20 Mar 2012 21:25:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13233791#comment-13233791
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Milind Bhandarkar commented on HDFS-3107:
-----------------------------------------

.bq Thats okay. You missed the smiley in the tweet too.

I just copy-pasted, so it was expected :-)

.bq I see I was not aware it was that common.

Since appends were enabled very recently, only those with the facebook's version of hadoop,
or hadoop 1.0 are users doing this now. Before this, users were creating multiple files.

In any case, my interest in this feature is for implementing transactions over HDFS (as Lei
and I have already discussed with Sanjay Radia and Hairong.) And aborting a transaction means
truncating to the last known good data across multiple files.

                
> 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
>
>   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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