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Date: Tue, 16 Jun 2009 12:57:08 -0700 (PDT)
From: "dhruba borthakur (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-5795) Add a bulk
 FIleSystem.getFileBlockLocations
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    [ https://issues.apache.org/jira/browse/HADOOP-5795?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12720318#action_12720318 ] 

dhruba borthakur commented on HADOOP-5795:
------------------------------------------

I think the extended version of the API would help in doing incremental distcp when hdfs-append is supported. We  use "distcp -update" to do an incremental copy of files that have changed in length, but having this proposed extended API (and more) allows distcp to copy only changed portions of a file. 


> Add a bulk FIleSystem.getFileBlockLocations
> -------------------------------------------
>
>                 Key: HADOOP-5795
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5795
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>    Affects Versions: 0.20.0
>            Reporter: Arun C Murthy
>            Assignee: Jakob Homan
>             Fix For: 0.21.0
>
>
> Currently map-reduce applications (specifically file-based input-formats) use FileSystem.getFileBlockLocations to compute splits. However they are forced to call it once per file.
> The downsides are multiple:
>    # Even with a few thousand files to process the number of RPCs quickly starts getting noticeable
>    # The current implementation of getFileBlockLocations is too slow since each call results in 'search' in the namesystem. Assuming a few thousand input files it results in that many RPCs and 'searches'.
> It would be nice to have a FileSystem.getFileBlockLocations which can take in a directory, and return the block-locations for all files in that directory. We could eliminate both the per-file RPC and also the 'search' by a 'scan'.
> When I tested this for terasort, a moderate job with 8000 input files the runtime halved from the current 8s to 4s. Clearly this is much more important for latency-sensitive applications...

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