[jira] [Created] (HDFS-13924) Handle BlockMissingException when reading from observer

["Chao Sun (JIRA)" <ji...@apache.org>]

Chao Sun created HDFS-13924:
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             Summary: Handle BlockMissingException when reading from observer
                 Key: HDFS-13924
                 URL: https://issues.apache.org/jira/browse/HDFS-13924
             Project: Hadoop HDFS
          Issue Type: Sub-task
            Reporter: Chao Sun


Internally we found that reading from ObserverNode may result to {{BlockMissingException}}. This may happen when the observer sees a smaller number of DNs than active (maybe due to communication issue with those DNs), or (we guess) late block reports from some DNs to the observer. This error happens in [DFSInputStream#chooseDataNode|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java#L846], when no valid DN can be found for the {{LocatedBlock}} got from the NN side.

One potential solution (although a little hacky) is to ask the {{DFSInputStream}} to retry active when this happens. The retry logic already present in the code - we just have to dynamically set a flag to ask the {{ObserverReadProxyProvider}} try active in this case.

cc [~shv], [~xkrogen], [~vagarychen], [~zero45] for discussion.



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