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https://issues.apache.org/jira/browse/HADOOP-1700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12523886
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Doug Cutting commented on HADOOP-1700:
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I think the easiest way to implement appends (and other file modifications) is
to keep blocks immutable, but permit one to alter the block list of a file.
The simplest implementation of this would require the client, when appending,
to re-read the entire last block of a file and then re-write a longer version
it as a new block. This could perhaps be optimized by adding a datanode
command that permitted one to start writing a new block on a datanode whose
initial contents is that of another block, then append new data to that block.
That datanode could chain the append to the other replicas.
> Append to files in HDFS
> -----------------------
>
> Key: HADOOP-1700
> URL: https://issues.apache.org/jira/browse/HADOOP-1700
> Project: Hadoop
> Issue Type: New Feature
> Components: dfs
> Reporter: stack
>
> Request for being able to append to files in HDFS has been raised a couple of
> times on the list of late. For one example, see
> http://www.nabble.com/HDFS%2C-appending-writes-status-tf3848237.html#a10916193.
> Other mail describes folks' workarounds because this feature is lacking:
> e.g. http://www.nabble.com/Loading-data-into-HDFS-tf4200003.html#a12039480
> (Later on this thread, Jim Kellerman re-raises the HBase need of this
> feature). HADOOP-337 'DFS files should be appendable' makes mention of file
> append but it was opened early in the life of HDFS when the focus was more on
> implementing the basics rather than adding new features. Interest fizzled.
> Because HADOOP-337 is also a bit of a grab-bag -- it includes truncation and
> being able to concurrently read/write -- rather than try and breathe new life
> into HADOOP-337, instead, here is a new issue focused on file append.
> Ultimately, being able to do as the google GFS paper describes -- having
> multiple concurrent clients making 'Atomic Record Append' to a single file
> would be sweet but at least for a first cut at this feature, IMO, a single
> client appending to a single HDFS file letting the application manage the
> access would be sufficent.
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