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https://issues.apache.org/jira/browse/HBASE-17565?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15851954#comment-15851954
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Enis Soztutar commented on HBASE-17565:
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I think we should add a couple of mock clusters which are like the cluster
described in the description, something like {{500, 500, 500, 500, 500, 0}},
{{1500, 500, 500, 500, 10, 0}}, {{1500, 500, 500, 10, 10, 0}}, etc.
TestStochasticLoadBalancer has very similar tests already, so we can extend the
tests to reproduce the case of needsBalance() not working correctly without the
patch, and fixing the test with the patch.
> StochasticLoadBalancer may incorrectly skip balancing due to skewed
> multiplier sum
> ----------------------------------------------------------------------------------
>
> Key: HBASE-17565
> URL: https://issues.apache.org/jira/browse/HBASE-17565
> Project: HBase
> Issue Type: Bug
> Reporter: Ted Yu
> Assignee: Ted Yu
> Priority: Critical
> Fix For: 2.0.0, 1.4.0
>
> Attachments: 17565.v1.txt, 17565.v2.txt
>
>
> I was investigating why a 6 node cluster kept skipping balancing requests.
> Here were the region counts on the servers:
> 449, 448, 447, 449, 453, 0
> {code}
> 2017-01-26 22:04:47,145 INFO
> [RpcServer.deafult.FPBQ.Fifo.handler=1,queue=0,port=16000]
> balancer.StochasticLoadBalancer: Skipping load balancing because balanced
> cluster; total cost is 127.0171157050385, sum multiplier is 111087.0 min cost
> which need balance is 0.05
> {code}
> The big multiplier sum caught my eyes. Here was what additional debug logging
> showed:
> {code}
> 2017-01-27 23:25:31,749 DEBUG
> [RpcServer.deafult.FPBQ.Fifo.handler=9,queue=0,port=16000]
> balancer.StochasticLoadBalancer: class
> org.apache.hadoop.hbase.master.balancer.
> StochasticLoadBalancer$RegionReplicaHostCostFunction with multiplier 100000.0
> 2017-01-27 23:25:31,749 DEBUG
> [RpcServer.deafult.FPBQ.Fifo.handler=9,queue=0,port=16000]
> balancer.StochasticLoadBalancer: class
> org.apache.hadoop.hbase.master.balancer.
> StochasticLoadBalancer$RegionReplicaRackCostFunction with multiplier 10000.0
> {code}
> Note however, that no table in the cluster used read replica.
> I can think of two ways of fixing this situation:
> 1. If there is no read replica in the cluster, ignore the multipliers for the
> above two functions.
> 2. When cost() returned by the CostFunction is 0 (or very very close to 0.0),
> ignore the multiplier.
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