distcp split generation does not work correctly
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Key: HADOOP-2032
URL: https://issues.apache.org/jira/browse/HADOOP-2032
Project: Hadoop
Issue Type: Bug
Components: util
Reporter: Runping Qi
With the current implementation, distcp will always assign multiple files to
one mapper to copy, no matter how large
are the files. This is because the CopyFiles class uses a sequencefile to store
the list of files to be copied,
one record per file. CopyFile class correctly generates one split per record in
the sequence file. However,
due to the way the sequence file record reader works, the minimum unit for
splits is the segments between the
"syncmarks" in the sequence file.
This results in the strange behavior that some mappers get zero records (zero
files to copy) even though their
split lengths are non-zero, while other mappers get multiple records (multiple
filesto copy) from their split (and beyond
to the next sync mark).
When CopyFile class creates the sequencefile, it does try to place a sync mark
between splitable segments in the sequence file by calling sync() function of
the sequence file record writer.
Unfortunately, the sync() function is a no-op for files that are not block
compressed.
Naturally, after I changed the compression type for the sequence file to block
compression,
mappers got the correct records from their splits.
So a simple fix is to change the compression tye to CompressionType.BLOCK:
{code}
// create src list
SequenceFile.Writer writer = SequenceFile.createWriter(
jobDirectory.getFileSystem(jobConf), jobConf, srcfilelist,
LongWritable.class, FilePair.class,
SequenceFile.CompressionType.BLOCK);.
{code}
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