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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3107) HDFS truncate
Date Wed, 01 Oct 2014 21:12:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14155537#comment-14155537
] 

Suresh Srinivas commented on HDFS-3107:
---------------------------------------

bq. In fact, if I were to rank the importance of snapshots vs. truncate to the majority of
our users, I'd say truncate functionality is way more important.
But the snapshots feature is already in the older releases (available since 2.1.0). All [~cmccabe]
is saying is, it is an existing feature and is currently used by many deployments. So debating
a feature that already exists vs. a new feature is not useful.

I agree with [~jingzhao]'s comment. We should address the interaction of this feature with
Snapshots. Having a feature that fails in unpredictable ways when a file has been snapshotted
vs. not is confusing. Postponing that will also result in an API that changes its behavior.

> 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
>         Attachments: HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch, HDFS-3107.patch,
HDFS-3107.patch, HDFS_truncate.pdf, HDFS_truncate.pdf, HDFS_truncate_semantics_Mar15.pdf,
HDFS_truncate_semantics_Mar21.pdf, editsStored
>
>   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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