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https://issues.apache.org/jira/browse/ACCUMULO-2488?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13938409#comment-13938409
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Bill Havanki commented on ACCUMULO-2488:
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Thanks, that's good information. There are actually five tables, as it turns
out.
Now, the question is, what should this balance check test for? Should it test
that the default balancer is working as expected? Or should it test that the
outcome fits some general definition of "balanced"? For example, that no server
should be more than _n_ standard deviations away from the mean.
> Concurrent randomwalk balance check needs refinement
> ----------------------------------------------------
>
> Key: ACCUMULO-2488
> URL: https://issues.apache.org/jira/browse/ACCUMULO-2488
> Project: Accumulo
> Issue Type: Test
> Components: test
> Affects Versions: 1.4.4
> Reporter: Bill Havanki
> Assignee: Bill Havanki
> Priority: Minor
> Labels: randomwalk, test
>
> The check for balanced tablets in the randomwalk Concurrent test too easily
> fails.
> Here is a real-life example from the test for the number of tablets across
> five tablet servers: 2, 5, 2, 2, 3. (An old unrelated table plays into these
> totals.) This produces a mean of 2.8. The cluster is considered unbalanced by
> the test when any server's count differs from the mean by the larger of 1 or
> the mean divided by 5. In this case, 2.8/5 is less than 1, so the second
> tablet server fails since it has more than 3.8 tablets. Even a 4 would fail.
> Part of the problem in this particular case is that there are so few tablets,
> and so few tablet servers. The cluster also seems happy to leave these counts
> as is, as I continue to check it, so the test's definition of unbalanced is
> too narrow.
> The test needs to be refined to detect unbalanced conditions with a
> statistically decent calculation.
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