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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