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Date: Wed, 11 Jun 2008 09:44:45 -0700 (PDT)
From: "Rong-En Fan (JIRA)" <jira@apache.org>
To: hbase-dev@hadoop.apache.org
Subject: [jira] Commented: (HBASE-615) Region balancer oscillates during
 cluster startup
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    [ https://issues.apache.org/jira/browse/HBASE-615?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12604268#action_12604268 ] 

Rong-En Fan commented on HBASE-615:
-----------------------------------

Please correct me if I am wrong or overlook something.

During startup, META will be requested more than other regions. Therefore,
the RegionServer that serves META will be considered more "loaded" than
others. So, we tends not to assign more regions to that one. However,
our rebalance algo currently considers only # of loaded regions as the "load"
for region servers. That's the cause of oscillation at startup.

I'm thinking of the possibility that during startup, we just use assign
evenly to all region servers. Once this is stabilized, we start to consider
# of requests as part of the server load. Moreover, the "# of requests" here
should be calculated from a period of time, otherwise, we may moving regions
just because some spikes.

just my 2 cents.

> 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
>            Priority: Blocker
>             Fix For: 0.2.0
>
>         Attachments: 615-lite.patch
>
>
> 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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