Ray Mattingly created HBASE-29070:
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             Summary: Balancer cost function epsilon is imprecise
                 Key: HBASE-29070
                 URL: https://issues.apache.org/jira/browse/HBASE-29070
             Project: HBase
          Issue Type: Bug
    Affects Versions: 2.6.1
            Reporter: Ray Mattingly
            Assignee: Ray Mattingly


The balancer cost function is imprecise. This means that the balancer can 
behave in unexpected ways at certain scales. For example, with enough regions 
and replicas, it's very possible for the 
StochasticLoadBalancers#areReplicasColocated (paraphrased) method to return a 
false negative, and fail to trigger a balancer run despite bad replica 
locations existing.

One immediate consequence of this bug is that our test suites are designed in 
slow and flappy ways. They all have huge balancer run times, but really our 
tests should be designed with tight balancer run times and iteration if 
necessary (in most cases). If we had done that, then it would've exposed this 
failure of the balancer to act as necessary, and our test suite would be much 
faster.



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