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From "Ted Yu (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3586) Randomize the selection of regions to balance
Date Tue, 01 Mar 2011 19:34:37 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13001047#comment-13001047
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Ted Yu commented on HBASE-3586:
-------------------------------

See my comment at 29/Jan/11 00:53 in HBASE-3373

> Randomize 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
>            Priority: Critical
>             Fix For: 0.90.2
>
>
> 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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