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From "Tom White (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-3095) Validating input paths and creating splits is slow on S3
Date Wed, 26 Mar 2008 17:31:30 GMT
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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