Clara Xiong created HBASE-25739:
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Summary: TableSkewCostFunction need to use aggregated deviation
Key: HBASE-25739
URL: https://issues.apache.org/jira/browse/HBASE-25739
Project: HBase
Issue Type: Sub-task
Components: Balancer, master
Reporter: Clara Xiong
TableSkewCostFunction uses the sum of the max deviation region per server for
all tables as the measure of unevenness. It doesn't work in a very common
scenario in operations. Say we have 100 regions on 50 nodes, two on each. We
add 50 new nodes and they have 0 each. The max deviation from the mean is 1,
compared to 99 in the worst case scenario of 100 regions on a single server.
The normalized cost is 1/99 = 0.011 < default threshold of 0.05. Balancer
wouldn't move. The proposal is to use the standard deviation of the count per
region server to detect this scenario, generating a cost of 3.1/31 = 0.1 in
this case.
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