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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-767) Job failure due to BlockMissingException
Date Thu, 12 Nov 2009 19:45:39 GMT
Job failure due to BlockMissingException
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                 Key: HDFS-767
                 URL: https://issues.apache.org/jira/browse/HDFS-767
             Project: Hadoop HDFS
          Issue Type: Improvement
            Reporter: Ning Zhang


If a block is request by too many mappers/reducers (say, 3000) at the same time, a BlockMissingException
is thrown because it exceeds the upper limit (I think 256 by default) of number of threads
accessing the same block at the same time. The DFSClient wil catch that exception and retry
3 times after waiting for 3 seconds. Since the wait time is a fixed value, a lot of clients
will retry at about the same time and a large portion of them get another failure. After 3
retries, there are about 256*4 = 1024 clients got the block. If the number of clients are
more than that, the job will fail. 

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