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https://issues.apache.org/jira/browse/HADOOP-3095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12584205#action_12584205
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Doug Cutting commented on HADOOP-3095:
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Shouldn't we deprecate validateInput() too?
Also, I second Tom's concern about breaking applications that override
listPaths(). If we assume that application code only overrides listPaths() but
doesn't call it, then we might change the default implementation to return a
special value, indicating that listPaths() has not been overridden. Then
listStatus() can call this and use the returned paths if the value is not the
special value, also emitting a warning that the method has been deprecated. If
we wanted to get really fancy, we could only return the special value when
called under listStatus() by setting a threadlocal, for 100%
back-compatibility. Ugly, huh?
> 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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