When DFS client fails to read from a datanode, the failed datanode is not
excluded from target reselection
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Key: HADOOP-698
URL: http://issues.apache.org/jira/browse/HADOOP-698
Project: Hadoop
Issue Type: Bug
Components: dfs
Reporter: Hairong Kuang
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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