Randomize the selection of regions to balance
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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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