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https://issues.apache.org/jira/browse/HDFS-5096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13741192#comment-13741192
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Bikas Saha commented on HDFS-5096:
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Will this automatic caching kick-in when the file is completely written or 
while writes are still in-progress? For MR etc it will be most beneficial when 
the file cache has an all or none property. That is complete files are present 
in the cache.
                
> Automatically cache new data added to a cached path
> ---------------------------------------------------
>
>                 Key: HDFS-5096
>                 URL: https://issues.apache.org/jira/browse/HDFS-5096
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: datanode, namenode
>            Reporter: Andrew Wang
>
> For some applications, it's convenient to specify a path to cache, and have 
> HDFS automatically cache new data added to the path without sending a new 
> caching request or a manual refresh command.
> One example is new data appended to a cached file. It would be nice to 
> re-cache a block at the new appended length, and cache new blocks added to 
> the file.
> Another example is a cached Hive partition directory, where a user can drop 
> new files directly into the partition. It would be nice if these new files 
> were cached.
> In both cases, this automatic caching would happen after the file is closed, 
> i.e. block replica is finalized.

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