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https://issues.apache.org/jira/browse/HBASE-3586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13003110#comment-13003110
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Ted Yu commented on HBASE-3586:
-------------------------------
Although random selection statistically achieves balance, I still prefer a
deterministic approach.
Consider the following variant for my patches:
Instead of using the following loop to fill out regionsToMove:
{code}
for (int i = sz-1; i >= 0; i--) {
HRegionInfo hri = regions.get(i);
{code}
We alternate between the head and tail of regions (picking both young and old
ones).
We still keep the following sort():
{code}
Collections.sort(regionsToMove, rpComparator);
{code}
We then iterate through underloadedServers repeatedly, doing the following
action alternately:
picking one region from head of regionsToMove in passes 1, 3, 5, etc
picking one region from tail of regionsToMove in passes 2, 4, 6, etc.
E.g. suppose RS1 has regions with region Ids of 42, 54, 105 and 201
RS2 has regions with region Ids of 34, 104, 110 and 154
Suppose we need to offload some regions to RS3 and RS4 which didn't carry
regions.
regionsToMove would contain regions (201, 42, 154, 34) before sorting and
regions (201, 154, 42, 34) after sorting.
Then we assign
region 201 to RS3, region 154 to RS4
region 34 to RS3, region 42 to RS4
Please comment.
> 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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