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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1595) DFSClient may incorrectly detect datanode failure
Date Mon, 24 Jan 2011 18:54:43 GMT
DFSClient may incorrectly detect datanode failure

                 Key: HDFS-1595
                 URL: https://issues.apache.org/jira/browse/HDFS-1595
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: data-node, hdfs client
    Affects Versions: 0.20.4
            Reporter: Tsz Wo (Nicholas), SZE

Suppose a source datanode S is writing to a destination datanode D in a write pipeline.  We
have an implicit assumption that _if S catches an exception when it is writing to D, then
D is faulty and S is fine._  As a result, DFSClient will take out D from the pipeline, reconstruct
the write pipeline with the remaining datanodes and then continue writing .

However, we find a case that the faulty machine F is indeed S but not D.  In the case we found,
F has a faulty network interface (or a faulty switch port) in such a way that the faulty network
interface works fine when sending out a small amount of data, say 1MB, but it fails when sending
out a large amount of data, say 100MB.

It is even worst if F is the first datanode in the pipeline.  Consider the following:
# DFSClient creates a pipeline with three datanodes.  The first of the datanode is F.
# F catches an IOException when writing to the second datanode. Then, F reports the second
datanode has error.
# DFSClient removes the second datanode from the pipeline and continue writing with the remaining
# The pipeline now has two datanodes but (2) and (3) repeat.
# Now, only F remains in the pipeline.  DFSClient continues writing with one replica in F.
# The write succeeds and DFSClient is able to close the file successfully.
# The block is under replicated.  The NameNode schedules replication from F to some other
datanode D.
# The replication fails from the same reason.  D reports to the NameNode that the replica
in F is corrupted.
# The NameNode marks the replica in F is corrupted.
# The block is corrupted since no replica is available.

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