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https://issues.apache.org/jira/browse/HADOOP-3095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12584058#action_12584058
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Tom White commented on HADOOP-3095:
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This looks good to me. A couple of points:
1. As it stands FileInputFormat#listPaths() is deprecated and not called.
However, this will silently change the behaviour of applications which have
overridden this method - and which are not recompiled - since it will never be
called. It would be nice to issue a warning at runtime, but I'm not sure how we
do this though, since it depends on being able to detect the existence of an
overridden method. Any thoughts?
2. There is still an FileSystem#exists() call in
FileSystem#getBlockLocations(). Could we change/overload the latter to be
FileSystem#getBlockLocations(FileStatus, long, long)?
> Validating input paths and creating splits is slow on S3
> --------------------------------------------------------
>
> Key: HADOOP-3095
> URL: https://issues.apache.org/jira/browse/HADOOP-3095
> Project: Hadoop Core
> Issue Type: Improvement
> Components: fs, fs/s3
> Reporter: Tom White
> Attachments: faster-job-init.patch
>
>
> A call to listPaths on S3FileSystem results in an S3 access for each file in
> the directory being queried. If the input contains hundreds or thousands of
> files this is prohibitively slow. This method is called in
> FileInputFormat.validateInput and FileInputFormat.getSplits. This would be
> easy to fix by overriding listPaths (all four variants) in S3FileSystem to
> not use listStatus which creates a FileStatus object for each subpath.
> However, listPaths is deprecated in favour of listStatus so this might be OK
> as a short term measure, but not longer term.
> But it gets worse: FileInputFormat.getSplits goes on to access S3 a further
> six times for each input file via these calls:
> 1. fs.isDirectory
> 2. fs.exists
> 3. fs.getLength
> 4. fs.getLength
> 5. fs.exists (from fs.getFileBlockLocations)
> 6. fs.getBlockSize
> So it would be best to change getSplits to use listStatus, and only access S3
> once for each file. (This would help HDFS too.) This change would require
> some care since FileInputFormat has a protected method listPaths which
> subclasses can override (although, in passing I notice validateInput doesn't
> use listPaths - is this a bug?).
> For input validation, one approach would be to disable it for S3 by creating
> a custom FileInputFormat. In this case, missing files would be detected
> during split generation. Alternatively, it may be possible to cache the input
> paths between validateInput and getSplits.
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