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From "Milind Bhandarkar (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-725) chooseTargets method in FSNamesystem is very inefficient
Date Wed, 15 Nov 2006 23:40:38 GMT
chooseTargets method in FSNamesystem is very inefficient
--------------------------------------------------------

                 Key: HADOOP-725
                 URL: http://issues.apache.org/jira/browse/HADOOP-725
             Project: Hadoop
          Issue Type: Improvement
          Components: dfs
    Affects Versions: 0.8.0
         Environment: All
            Reporter: Milind Bhandarkar
         Assigned To: Milind Bhandarkar
             Fix For: 0.9.0


Currently the chooseTargets method (that selects datanodes for block-placement) takes in excess
of 20% of cpu on a namenode. This is the most time-consuming namenode method, according to
the profiler. This inefficiency has already contributed to cascading crash in DFS earlier.
As datanodes went down, new locations needed to be found for the blocks on dead datanodes,
and since this was done inside a synchronized method, it locked the whole namesystem for several
minutes, which caused more datanode failures, when the namenode marked them dead because no
heartbeat could be processed during that interval. This has been detailed in HADOOP-572.

The patch I am about to upload reduces the time taken in the chooseTarget method to be proportional
to nReplicas per block, instead of the current implementation, which is proportional to (nDataNodes
* nReplicas). Also, when a number of datanodes crash, their blocks are put on the pendingReplications
list one datanode at a time in a synchronized section. (Currently, the syncchronized section
processes ALL the dead datanodes, thus locking the namesystem for a considerable amount of
time.) Also, this patch will add a unit test to check replication.


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