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https://issues.apache.org/jira/browse/HDFS-202?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12893883#action_12893883
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Suresh Srinivas commented on HDFS-202:
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Unix 'ls' returns all the results in one shot. However, when getting response
iteratively the behavior is different:
# When listing a single directory, if some ls results has been returned and the
directory is deleted, we should throw FileNotFoundException, to indicate the
directory is no longer available.
# When recursively listing under a directory, if a subdirectory is deleted, the
more appropriate response is to ignore FileNotFound for that directory and
return the remaining results. This would be consistent with what the result
would be, if the command is repeated. Further, if an application is listing
recursively a large directory, the state of the directory keeps changing, an
application may have to try many times to list it.
> 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: Hairong Kuang
> Fix For: 0.22.0
>
> Attachments: hdfsListFiles.patch, hdfsListFiles1.patch
>
>
> 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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