DFS should place one replica per rack
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Key: HADOOP-2559
URL: https://issues.apache.org/jira/browse/HADOOP-2559
Project: Hadoop
Issue Type: Improvement
Components: dfs
Reporter: Runping Qi
Currently, when writing out a block, dfs will place one copy to a local data
node, one copy to a rack local node
and another one to a remote node. This leads to a number of undesired
properties:
1. The block will be rack-local to two tacks instead of three, reducing the
advantage of rack locality based scheduling by 1/3.
2. The Blocks of a file (especiallya large file) are unevenly distributed over
the nodes: One third will be on the local node, and two thirds on the nodes on
the same rack. This may make some nodes full much faster than others,
increasing the need of rebalancing. Furthermore, this also make some nodes
become "hot spots" if those big
files are popular and accessed by many applications.
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