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https://issues.apache.org/jira/browse/HBASE-71?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12572294#action_12572294
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Bryan Duxbury commented on HBASE-71:
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In a naive approach, the RegionManager decides that a regionserver is
overloaded and tells it to close some of its regions, puts them back in the
unassigned region list, and lets them get randomly reassigned. The problem with
this approach is that during the time when it's being "transferred" (offline
pending reassignment), the region will be unavailable. That would be sort of
undesirable, since it would mean downtime and possibly a lot of retries for
some consumers.
Instead, maybe we could introduce a way for one regionserver to keep serving a
region while another opens it read-only. Then, once the second server has it
opened, we could switch who is assigned as the writer server. This would
introduce a period of time where there might not be consistent state because of
the memcache on the original writer. Not sure this would work, but it could
help.
> [hbase] Master should rebalance region assignments periodically
> ---------------------------------------------------------------
>
> Key: HBASE-71
> URL: https://issues.apache.org/jira/browse/HBASE-71
> Project: Hadoop HBase
> Issue Type: Improvement
> Components: master
> Reporter: Bryan Duxbury
> Assignee: Bryan Duxbury
> Fix For: 0.2.0
>
>
> The master currently only does region assignments at startup or when there
> are splits or dead regionservers. This means that if you join a new
> regionserver to the cluster after startup, it does not get assigned a fair
> share of the already-served regions as you would expect. It only gets a share
> of new regions being served.
> The master should periodically check the balance of regions, based on
> whatever assignment function, instead of in reaction to the above listed
> events.
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