[ http://issues.apache.org/jira/browse/HADOOP-698?page=all ]
Milind Bhandarkar updated HADOOP-698:
-------------------------------------
Status: Patch Available (was: In Progress)
> When DFS client fails to read from a datanode, the failed datanode is not
> excluded from target reselection
> ----------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-698
> URL: http://issues.apache.org/jira/browse/HADOOP-698
> Project: Hadoop
> Issue Type: Bug
> Components: dfs
> Reporter: Hairong Kuang
> Assigned To: Milind Bhandarkar
> Attachments: datanode-exclude.patch
>
>
> In the method read(byte buf[ ], int off, int len) of DFSInputStream, when
> read fails, it calls "blockSeekTo" to reselect a datanode. However, the
> failed datanode does not feed back to blockSeekTo. The datanode selection
> algorithm works as follows:
> * If the machine that the client is running on has a local copy, return the
> local machine;
> * Otherwise, randomly pick up one location.
> When the failed data node info does not feed back to target reselection, this
> leads to two flaws:
> 1. When a client fails to read from the local copy, for example, because of
> the checksum error, the local machine will always be chosen in retries.
> 2. Random selection may still return the same failed node.
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