Bryan Beaudreault created HDFS-5837:
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Summary: dfs.namenode.replication.considerLoad does not consider
decommissioned nodes
Key: HDFS-5837
URL: https://issues.apache.org/jira/browse/HDFS-5837
Project: Hadoop HDFS
Issue Type: Bug
Components: namenode
Reporter: Bryan Beaudreault
In DefaultBlockPlacementPolicy, there is a setting
dfs.namenode.replication.considerLoad which tries to balance the load of the
cluster when choosing replica locations. This code does not take into account
decommissioned nodes.
The code for considerLoad calculates the load by doing: TotalClusterLoad /
numNodes. However, numNodes includes decommissioned nodes (which have 0 load).
Therefore, the average load is artificially low. Example:
TotalLoad = 250
numNodes = 100
decommissionedNodes = 50
avgLoad = 250/100 = 2.50
trueAvgLoad = 250 / (100 - 70) = 8.33
If the real load of the remaining 30 nodes is (on average) 8.33, this is more
than 2x the calculated average load of 2.50. This causes these nodes to be
rejected as replica locations. The final result is that all nodes are rejected,
and no replicas can be placed.
See exceptions printed from client during this scenario:
https://gist.github.com/bbeaudreault/49c8aa4bb231de54e9c1
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