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https://issues.apache.org/jira/browse/HADOOP-3095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12583917#action_12583917
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Doug Cutting commented on HADOOP-3095:
--------------------------------------
FileInputFormat#getSplits() in trunk still calls FileSystem#exists(),
FileSystem#isDirectory(), FileSystem#getLength() (twice) and
FileSystem#getBlockSize() for each input file. These could all be eliminated
if it used listStatus() instead of listPaths(). The cost of these is
negligible for HDFS currently, since these are all cached in the Path instance.
But once we eliminate that cache, performance on HDFS will suffer.
FileSystem#getBlockLocations() is also called once per file and would still
need to be called, since FileStatus doesn't (yet) include that information.
Also, do you see a reason for validateInput to continue to exist? Without the
status cache it too will become very slow, and its purpose is now outdated.
> 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
>
> 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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