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From "Kahlil Oppenheimer (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17707) New More Accurate Table Skew cost function/generator
Date Fri, 17 Mar 2017 15:31:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15930137#comment-15930137
] 

Kahlil Oppenheimer commented on HBASE-17707:
--------------------------------------------

The issue here is that the tests are checking to make sure no two replicas of the same region
end up on the same host, but that constraint is only enforced by having a very high weight
associated with the RegionReplicaHost cost function. The issue with this is that even with
a very high weight (like 5000), the cost value for this function can get really small (like
.0000000026) as the number of regions grows large. Thus, the balancer might decide to move
a replica of a region to the same host as another because it benefits other cost functions
(like table skew) because the RegionReplicaHost cost is so small. I have two solutions that
would fix this:

1) Disable table skew generator/cost function when there are region replicas.

2) Change the RegionReplicaHost cost function to make the cost super high for any amount of
replicas on the same host, regardless of how many regions are in the cluster.

What are your thoughts?

> New More Accurate Table Skew cost function/generator
> ----------------------------------------------------
>
>                 Key: HBASE-17707
>                 URL: https://issues.apache.org/jira/browse/HBASE-17707
>             Project: HBase
>          Issue Type: New Feature
>          Components: Balancer
>    Affects Versions: 1.2.0
>         Environment: CentOS Derivative with a derivative of the 3.18.43 kernel. HBase
on CDH5.9.0 with some patches. HDFS CDH 5.9.0 with no patches.
>            Reporter: Kahlil Oppenheimer
>            Assignee: Kahlil Oppenheimer
>            Priority: Minor
>             Fix For: 2.0
>
>         Attachments: HBASE-17707-00.patch, HBASE-17707-01.patch, HBASE-17707-02.patch,
HBASE-17707-03.patch, HBASE-17707-04.patch, HBASE-17707-05.patch, HBASE-17707-06.patch, HBASE-17707-07.patch,
HBASE-17707-08.patch, HBASE-17707-09.patch, test-balancer2-13617.out
>
>
> This patch includes new version of the TableSkewCostFunction and a new TableSkewCandidateGenerator.
> The new TableSkewCostFunction computes table skew by counting the minimal number of region
moves required for a given table to perfectly balance the table across the cluster (i.e. as
if the regions from that table had been round-robin-ed across the cluster). This number of
moves is computer for each table, then normalized to a score between 0-1 by dividing by the
number of moves required in the absolute worst case (i.e. the entire table is stored on one
server), and stored in an array. The cost function then takes a weighted average of the average
and maximum value across all tables. The weights in this average are configurable to allow
for certain users to more strongly penalize situations where one table is skewed versus where
every table is a little bit skewed. To better spread this value more evenly across the range
0-1, we take the square root of the weighted average to get the final value.
> The new TableSkewCandidateGenerator generates region moves/swaps to optimize the above
TableSkewCostFunction. It first simply tries to move regions until each server has the right
number of regions, then it swaps regions around such that each region swap improves table
skew across the cluster.
> We tested the cost function and generator in our production clusters with 100s of TBs
of data and 100s of tables across dozens of servers and found both to be very performant and
accurate.



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