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From "Bryan Pendleton (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-429) Periodically move blocks from full nodes to those with space
Date Mon, 07 Aug 2006 22:22:14 GMT
 Periodically move blocks from full nodes to those with space
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                 Key: HADOOP-429
                 URL: http://issues.apache.org/jira/browse/HADOOP-429
             Project: Hadoop
          Issue Type: Improvement
            Reporter: Bryan Pendleton


Continuance of Hadoop-386. The patch to that issue makes it possible to redistribute blocks
(change replication up, wait for replication to succeed, then lower replication again). However,
this requires a lot more space, is not automatic, and doesn't respect a reasonable I/O limit.
I have actually had MapReduce jobs fail from block missing execptions after having recently
changed the replication level (from 3 to 4, with no underreplications to start with) because
the datanodes were too slow responding to requests while performing the necessary replications.

A good fix to this problem would be a low-priority thread on the NameNode that schedules low-priority
replications of blocks on over-full machines, followed by the removal of the extra replications.
It might be worth having a specific prototocol for asking for these low-priority copies to
happen in the datanodes, so that they continue to service (and be available to service) normal
block requests.

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