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https://issues.apache.org/jira/browse/HADOOP-9022?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13494685#comment-13494685
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Allen Wittenauer commented on HADOOP-9022:
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When -m 0 is specified, it should probably just call copy in a serial fashion
across the list and not run a MapReduce job. After all, setting the number of
maps to zero implies that one doesn't want a job executed at all.
> Hadoop distcp tool fails to copy file if -m 0 specified
> -------------------------------------------------------
>
> Key: HADOOP-9022
> URL: https://issues.apache.org/jira/browse/HADOOP-9022
> Project: Hadoop Common
> Issue Type: Bug
> Affects Versions: 0.23.1, 0.23.3, 0.23.4
> Reporter: Haiyang Jiang
>
> When trying to copy file using distcp on H23, if -m 0 is specified, distcp
> will just spawn 0 mapper tasks and the file will not be copied.
> But this used to work before H23, even when -m 0 specified, distcp will
> always copy the files.
> Checked the code of DistCp.java
> Before the rewrite, it set the number maps at least to 1
> job.setNumMapTasks(Math.max(numMaps, 1));
> But in the newest code, it just takes the input from user:
> job.getConfiguration().set(JobContext.NUM_MAPS,
> String.valueOf(inputOptions.getMaxMaps()));
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