[
https://issues.apache.org/jira/browse/HDFS-3107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14178044#comment-14178044
]
Plamen Jeliazkov commented on HDFS-3107:
----------------------------------------
[~srivas],
There is no plan to grow the file by padding it with zeroes as general-purpose
truncate does. Both [~shv] and [~lei_chang] mentioned this in their design
docs, I believe.
[~cmccabe],
While the copying the last block up to its truncate point and doing a
delete/concat is definitely a simpler overall approach, the full truncate
implementation has the benefit of being a single NameNode RPC call that can
both truncate in-place and copy-on-truncate, preserving the original last block
and moving the 'copy&truncate' work to the DataNodes themselves (as opposed to
having to pass data through the network / client). I am not intending to debate
either implementation -- I like both personally; just wanted to explain as
briefly as I could why Konstantin and I are taking our approach.
> 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-3107.008.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_semantics_Mar15.pdf, HDFS_truncate_semantics_Mar21.pdf,
> editsStored, 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
