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https://issues.apache.org/jira/browse/HDFS-202?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753215#action_12753215
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Doug Cutting commented on HDFS-202:
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> Is the optimization for sending only partial block reports really necessary?
It may help the append-savvy distcp use case, but is not in the mapred job
submission use case. Even in the append-savvy distcp use case, it's not clear
that it's required. Maybe we should punt that until someone develops an
append-savvy distcp?
> Why not create a class called DetailedFileStatus which contains both the file
> status and block locations:
Why is DetailedFileStatus[] better than Map<FileStatus,BlockLocation[]>? The
latter seems more transparent.
> DetailedFileStatus[] = getBlockLocations(Path[] paths); // 1:1 mapping
> between the two arrays as Doug suggested.
That was intended for the append-savvy distcp use case. The original use case
was for mapred job submission, where we typically have a list of directories.
With directories there is not a 1:1 mapping.
> Add a bulk FIleSystem.getFileBlockLocations
> -------------------------------------------
>
> Key: HDFS-202
> URL: https://issues.apache.org/jira/browse/HDFS-202
> Project: Hadoop HDFS
> Issue Type: New Feature
> Reporter: Arun C Murthy
> Assignee: Jakob Homan
>
> 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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