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https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13236370#comment-13236370
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Lei Chang commented on HDFS-3107:
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> Lei, do you see any issues with the proposal (i.e option 2) ?
I do like the second option :), for its simplicity to implement atomicity. I
also think it is better to not change the API which should be much easier to
use for end users. truncate(file, length, concatFile) can be used internally.
For the third option (1), it indeed introduces a lot of difficulties in
implementation, such as fault tolerance implementation issues. This has been
seen in our first try to implement truncate which takes a more strong semantic.
IMO, for visibility, the weak consistency for concurrent read is ok for upper
layer applications. For instance, most database systems already use their own
locks to synchronize the concurrent access to files or file blocks.
> 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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