FAQ
SequenceFile.Sorter creates local temp files on HDFS
----------------------------------------------------

Key: HADOOP-7611
URL: https://issues.apache.org/jira/browse/HADOOP-7611
Project: Hadoop Common
Issue Type: Bug
Components: io
Affects Versions: 0.20.2
Environment: CentOS 5.6 64-bit, Oracle JDK 1.6.0_26 64-bit
Reporter: Bryan Keller


When using SequenceFile.Sorter to sort or merge sequence files that exist in HDFS, it attempts to create temp files in a directory structure specified by mapred.local.dir but on HDFS, not in the local file system. The problem code is in MergeQueue.merge(). Starting at line 2953:
{code}
Path outputFile = lDirAlloc.getLocalPathForWrite(
tmpFilename.toString(),
approxOutputSize, conf);
LOG.debug("writing intermediate results to " + outputFile);
Writer writer = cloneFileAttributes(
fs.makeQualified(segmentsToMerge.get(0).segmentPathName),
fs.makeQualified(outputFile), null);
{code}
The outputFile here is a local path without a scheme, e.g. "/mnt/mnt1/mapred/local", specified by the mapred.local.dir property. If we are sorting files on HDFS, the fs object is a DistributedFileSystem. The call to fs.makeQualified(outputFile) appends the fs object's scheme to the local temp path returned by lDirAlloc, e.g. hdfs:///mnt/mnt1/mapred/local. This directory is then created (if the proper permissions are available) on HDFS. If the HDFS permissions are not available, the sort/merge fails even though the directories exist locally.

The code should instead always use the local file system if retrieving a path from the mapred.local.dir property. The unit tests do not test this condition, they only test using the local file system for sort and merge.



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postedSep 5, '11 at 5:46p
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