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From "Jim Kellerman (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-615) Region balancer oscillates during cluster startup
Date Tue, 06 May 2008 01:33:55 GMT
Region balancer oscillates during cluster startup

                 Key: HBASE-615
                 URL: https://issues.apache.org/jira/browse/HBASE-615
             Project: Hadoop HBase
          Issue Type: Bug
          Components: master
    Affects Versions: 0.2.0
            Reporter: Jim Kellerman
            Assignee: Bryan Duxbury

When starting a cluster with four region servers and a large table (49 regions) (+root +meta)
= 51 total regions, the region balancer oscillates for a very long time and does not seem
to reach a steady state.

Additionally, for whatever reason, it seems reluctant to assign regions to the first of four
region servers, which may be the root cause. In my test, the first server had 10 regions assigned,
the second and fourth had 13 regions assigned, and the master would continually assign and
deassign 2 regions to the third server, which oscillated between 13 and 15 regions.  If it
assigned the two fluctuating regions to the first server, it would achieve the best balance
possible: 12, 13, 13, 13.

After 20 minutes, it had not stopped oscillating. An application trying to work against this
cluster would run very slowly as it would be continually re-finding the two regions in flux.

When the table was being created, regions were nicely balanced. On restart, however, it just
would not settle down.

Perhaps the balancer should set a target number of regions for each server which when the
server achieved +/- 1 regions, the rebalancer would not try to change unless the number of
regions changed.

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