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From "Allan Yang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-17969) Balance by table using SimpleLoadBalancer could end up imbalance
Date Thu, 27 Apr 2017 08:53:04 GMT
Allan Yang created HBASE-17969:
----------------------------------

             Summary: Balance by table using SimpleLoadBalancer could end up imbalance
                 Key: HBASE-17969
                 URL: https://issues.apache.org/jira/browse/HBASE-17969
             Project: HBase
          Issue Type: Improvement
    Affects Versions: 1.1.10
            Reporter: Allan Yang
            Assignee: Allan Yang


This really happens in our production env.
Here is a example:
Say we have three RS named r1, r2, r3. A table named table1 with 3 regions distributes on
these rs like this:
r1 1
r2 1
r3 1
Each rs have one region, it means table1 is balanced. So balancer will not run.

If the region on r3 splits, then it becomes:
r1 1
r2 1
r3 2
For table1, in average, each rs will have min=1, max=2 regions. So still it is balanced, balancer
will not run.

Then a region on r3 splits again, the distribution becomes:
r1 1
r2 1
r3 3
In average, each rs will have min=1, max=2 regions. So balancer will run.
For r1 and r2, they have already have min=1 regions. Balancer won't do any operation on them.
But for r3, it exceed max=3, so balancer will remove one region from r3 and choose one rs
from r1, r2 to move to.
But r1 and r2 have the same load, so balancer will always choose r1 since servername r1 <
r2(alphabet order, sorted by ServerAndLoad's compareTo method). It is OK for table1 itself.
But if every table in the cluster have similar situations like table1, then the load in the
cluster will always be like r1 > r2 > r3.  
So, the solution here is when each rs reach min regions (min=total region / servers), but
there still some region need to move, shuffle the regionservers before move.



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