[HADOOP-3095] Validating input paths and creating splits is slow on S3

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.18.0
• Component/s: fs, fs/s3
• Hadoop Flags: Incompatible change, Reviewed

Release Note

Added overloaded method getFileBlockLocations(FileStatus, long, long). This is an incompatible change for FileSystem implementations which override getFileBlockLocations(Path, long, long). They should have the signature of this method changed to getFileBlockLocations(FileStatus, long, long) to work correctly.

• Assignee: Thomas White
• Reporter: Thomas White
• Created: 26/Mar/08 17:31
• Updated: 02/May/13 02:29
• Resolved: 04/Jun/08 20:31

Description

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.

Issue Links

• is depended upon by: HADOOP-2565 DFSPath cache of FileStatus can become stale
• is related to: HADOOP-3664 Remove deprecated methods introduced in changes to validating input paths (HADOOP-3095)