[ http://issues.apache.org/jira/browse/HADOOP-429?page=all ]
Sameer Paranjpye updated HADOOP-429:
------------------------------------
Component/s: dfs
> Periodically move blocks from full nodes to those with space
> -------------------------------------------------------------
>
> Key: HADOOP-429
> URL: http://issues.apache.org/jira/browse/HADOOP-429
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> 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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