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Date: Sat, 5 Mar 2011 23:25:45 +0000 (UTC)
From: "Ted Yu (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
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Subject: [jira] Commented: (HBASE-3586) Improve the selection of regions to
 balance
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    [ https://issues.apache.org/jira/browse/HBASE-3586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13003065#comment-13003065 ] 

Ted Yu commented on HBASE-3586:
-------------------------------

Although balanceCluster() can be made more complex by considering both old and new regions, the new patch achieves the same effect.
When creating a table with multiple regions, I check whether there're online region servers which don't carry region (this can be relaxed by introducing a threshold which separates overloaded and underloaded servers). If there're such servers, balance() is called to balance the (relatively old) regions.
Since assignmentManager.assignUserRegions() uses round-robin assignment, cluster would still be balanced when createTable() returns.

> Improve the selection of regions to balance
> -------------------------------------------
>
>                 Key: HBASE-3586
>                 URL: https://issues.apache.org/jira/browse/HBASE-3586
>             Project: HBase
>          Issue Type: Improvement
>    Affects Versions: 0.90.1
>            Reporter: Jean-Daniel Cryans
>            Assignee: Ted Yu
>            Priority: Critical
>             Fix For: 0.90.2
>
>         Attachments: HBASE-3586-by-region-age.patch, HBASE-3586-by-region-age.patch, hbase-3586-table-creation.txt, hbase-3586-with-sort.txt
>
>
> Currently LoadBalancer goes through the list of regions per RS and grabs the few first ones to balance. This is not bad, but that list is often sorted naturally since the a RS that boots will open the regions in a sequential and sorted order (since it comes from .META.) which means that we're balancing regions starting in an almost sorted fashion.
> We discovered that because one of our internal users created a new table starting with letter "p" which has now grown to 100 regions in the last few hours and they are all served by 1 region server. Looking at the master's log, the balancer has moved as many regions from that region server but they are all from the same table that starts with letter "a" (and the regions that were moved all come one after the other).
> The part of the code that should be modified is:
> {code}
> for (HRegionInfo hri: regions) {
>   // Don't rebalance meta regions.
>   if (hri.isMetaRegion()) continue; 
>   regionsToMove.add(new RegionPlan(hri, serverInfo, null));
>   numTaken++;
>   if (numTaken >= numToOffload) break;
> }
> {code}

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