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https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14288936#comment-14288936
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Konstantin Shvachko commented on HDFS-3107:
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Hey guys, Plamen's ports to branch-2 look good to me.
I'll schedule time during next weekend to commit these and related jiras. This 
is almost two weeks from the trunk commit.
LMK if there are any issues. Looks like we are making good progress.
I think we've reached the stage when further truncate related changes can be 
done both on trunk and branch-2 after the merge.
Nichloas, are you ok doing HDFS-7058, which has only TestCLI remaining, after 
the merge?

> HDFS truncate
> -------------
>
>                 Key: HDFS-3107
>                 URL: https://issues.apache.org/jira/browse/HDFS-3107
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: datanode, namenode
>            Reporter: Lei Chang
>            Assignee: Plamen Jeliazkov
>             Fix For: 3.0.0
>
>         Attachments: HDFS-3107-13.patch, HDFS-3107-14.patch, 
> HDFS-3107-15.patch, HDFS-3107-HDFS-7056-combined.patch, HDFS-3107.008.patch, 
> HDFS-3107.15_branch2.patch, HDFS-3107.patch, HDFS-3107.patch, 
> HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, 
> HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, 
> HDFS-3107.patch, HDFS_truncate.pdf, HDFS_truncate.pdf, HDFS_truncate.pdf, 
> HDFS_truncate_semantics_Mar15.pdf, HDFS_truncate_semantics_Mar21.pdf, 
> editsStored, editsStored.xml
>
>   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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