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From "Dmytro Molkov (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1105) Balancer improvement
Date Thu, 22 Apr 2010 01:59:49 GMT
Balancer improvement

                 Key: HDFS-1105
                 URL: https://issues.apache.org/jira/browse/HDFS-1105
             Project: Hadoop HDFS
          Issue Type: Improvement
            Reporter: Dmytro Molkov

We were seeing some weird issues with the balancer in our cluster:

1) it can get stuck during an iteration and only restarting it helps
2) the iterations are highly inefficient. With 20 minutes iteration it moves 7K blocks a minute
for the first 6 minutes and hundreds of blocks in the next 14 minutes
3) it can hit namenode and the network pretty hard

A few improvements we came up with as a result:
Making balancer more deterministic in terms of running time of iteration, improving the efficiency
and making the load configurable:

Make many of the constants configurable command line parameters: Iteration length, number
of blocks to move in parallel to a given node and in cluster overall.
Terminate transfers that are still in progress after iteration is over.

Previously iteration time was the time window in which the balancer was scheduling the moves
and then it would wait for the moves to finish indefinitely. Each scheduling task can run
up to iteration time or even longer. This means if you have too many of them and they are
long your actual iterations are longer than 20 minutes. Now each scheduling task has a time
of the start of iteration and it should schedule the moves only if it did not run out of time.
So the tasks that have started after the iteration is over will not schedule any moves.

The number of move threads and dispatch threads is configurable so that depending on the load
of the cluster you can run it slower.

I will attach a patch, please let me know what you think and what can be done better.

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