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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1595) DFSClient may incorrectly detect datanode failure
Date Mon, 24 Jan 2011 21:23:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1595?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12986035#action_12986035
] 

Tsz Wo (Nicholas), SZE commented on HDFS-1595:
----------------------------------------------

{quote}
Could we solve this with a blacklist-like feature? ie when a pipeline detects an issue, it
sends the pair (S, D) to the NameNode....
{quote}
This is a good idea to identify faulty datanodes.  However, it does not prevent data loss.

{quote}
Come to think of it, isn't that what dfs.replication.min does ...
{quote}
There is a subtle difference: if we set dfs.replication.min = 2, you are right that DFSClient.close()
will wait till two replicas reported to the NameNode.  However, all pipelines, healthy or
faulty, have to wait for two replicas.  Consequently, the normal cases are penalized and degrade
the performance.


> 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
>            Priority: Critical
>
> 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 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 datanode(s).
> # 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.
> This is a *data loss* scenario.

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