DFS Scalability: avoid scanning entire list of datanodes in getAdditionalBlocks
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Key: HADOOP-1187
URL: https://issues.apache.org/jira/browse/HADOOP-1187
Project: Hadoop
Issue Type: Bug
Components: dfs
Reporter: dhruba borthakur
Assigned To: dhruba borthakur
A new block allocations for a file scans the list of all known datanodes to
find if the client that is a also a cluster node. If so, then it tries to
allocate a replica locally. This check consumes plenty of CPU, especially if
the number of datanodes in a cluster is large.
An optimization: if the client is also a cluster node, then cache a reference
to the corresponding DatanodeDescriptor from the entry in pendingCreate. The
method getAdditionalBlock() uses the cached DatanodeDescriptor and thus avoids
scanning the entire list of datanodes.
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