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From "Ted Yu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4114) Metrics for HFile HDFS block locality
Date Wed, 20 Jul 2011 23:47:57 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4114?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13068724#comment-13068724
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Ted Yu commented on HBASE-4114:
-------------------------------

Maybe in phase 2, efficiently calculating locality index per region for a given region server
would be useful for load balancer to determine the best region server for the underlying region.

> Metrics for HFile HDFS block locality
> -------------------------------------
>
>                 Key: HBASE-4114
>                 URL: https://issues.apache.org/jira/browse/HBASE-4114
>             Project: HBase
>          Issue Type: Improvement
>          Components: metrics, regionserver
>            Reporter: Ming Ma
>            Assignee: Ming Ma
>
> Normally, when we put hbase and HDFS in the same cluster ( e.g., region server runs on
the datenode ), we have a reasonably good data locality, as explained by Lars. Also Work has
been done by Jonathan to address the startup situation.
> There are scenarios where regions can be on a different machine from the machines that
hold the underlying HFile blocks, at least for some period of time. This will have performance
impact on whole table scan operation and map reduce job during that time.
> 1.	After load balancer moves the region and before compaction (thus generate HFile on
the new region server ) on that region, HDFS block can be remote.
> 2.	When a new machine is added, or removed, Hbase's region assignment policy is different
from HDFS's block reassignment policy.
> 3.	Even if there is no much hbase activity, HDFS can load balance HFile blocks as other
non-hbase applications push other data to HDFS.
> Lots has been or will be done in load balancer, as summarized by Ted. I am curious if
HFile HDFS block locality should be used as another factor here.
> I have done some experiments on how HDFS block locality can impact map reduce latency.
First we need to define a metrics to measure HFile data locality.
> Metrics defintion:
> For a given table, or a region server, or a region, we can define the following. The
higher the value, the more local HFile is from region server's point of view.
> HFile locality index = ( Total number of HDFS blocks that can be retrieved locally by
the region server ) / ( Total number of HDFS blocks for all HFiles )
> Test Results:
> This is to show how HFile locality can impact the latency. It is based on a table with
1M rows, 36KB per row; regions are distributed in balance. The map job is RowCounter.
> HFile Locality Index	Map job latency ( in sec )
> 28%	                157
> 36%	                150
> 47%	                142
> 61%	                133
> 73%	                122
> 89%	                103
> 99%	                95
> So the first suggestion is to expose HFile locality index as a new region server metrics.
It will be ideal if we can somehow measure HFile locality index on a per map job level.
> Regarding if/when we should include that as another factor for load balancer, that will
be a different work item. It is unclear how load balancer can take various factors into account
to come up with the best load balancer strategy.

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