Sean Busbey created HBASE-24139:
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Summary: Balancer should avoid leaving idle region servers
Key: HBASE-24139
URL: https://issues.apache.org/jira/browse/HBASE-24139
Project: HBase
Issue Type: Improvement
Components: Balancer, Operability
Reporter: Sean Busbey
Assignee: Sean Busbey
After HBASE-15529 the StochasticLoadBalancer makes the decision to run based on
its internal cost functions rather than the simple region count skew of
BaseLoadBalancer.
Given the default weights for those cost functions, the default minimum cost to
indicate a need to rebalance, and a regions per region server density of ~90 we
are not very responsive to adding additional region servers for non-trivial
cluster sizes:
* For clusters ~10 nodes, the defaults think a single RS at 0 regions means we
need to balance
* For clusters >20 nodes, the defaults will not consider a single RS at 0
regions to mean we need to balance. 2 RS at 0 will cause it to balance.
* For clusters ~100 nodes, having 6 RS with no regions will still not meet the
threshold to cause a balance.
Note that this is the decision to look at balancer plans at all. The
calculation is severely dominated by the region count skew (it has weight 500
and all other weights are ~105), so barring a very significant change in all
other cost functions this condition will persist indefinitely.
Two possible approaches:
* add a new cost function that's essentially "don't have RS with 0 regions"
that an operator can tune
* add a short circuit condition for the {{needsBalance}} method that checks for
empty RS similar to the check we do for colocated region replicas
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