[jira] Commented: (HADOOP-5795) Add a bulk FIleSystem.getFileBlockLocations

["Doug Cutting (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-5795?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12707433#action_12707433 ] 

Doug Cutting commented on HADOOP-5795:
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An alternative to passing directories might be to pass a list of files.  The request might get larger, but this is more precise, e.g., when only a subset of files in a directory will be used only that subset need be passed.  Since globbing is client-side, this requires two round trips, one to list files and one to list their blocks, but that would still be a huge improvement over per-file RPC.

> 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
>             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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