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From "Bryan Duxbury (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-71) [hbase] Master should rebalance region assignments periodically
Date Mon, 25 Feb 2008 22:53:51 GMT

    [ https://issues.apache.org/jira/browse/HBASE-71?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12572294#action_12572294

Bryan Duxbury commented on HBASE-71:

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

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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