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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6072) distcp should place the file distcp_src_files in distributed cache
Date Thu, 18 Jun 2009 19:58:07 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6072?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12721437#action_12721437
] 

Doug Cutting commented on HADOOP-6072:
--------------------------------------

> Is that still OK fornamenode's perf ?

This should not be a problem for the namenode.  It would be best to write the file first with
normal replication, then increase its replication, to avoid an overly-long HDFS write pipeline.

The rationale for sqrt is that a two-stage fanout is done: first from the original to the
replicas, then from the replicas to the maps.  Sqrt(maps) uses approximately the same fanout
factor at each stage, minimizing the number of datanode clients (the presumed bottleneck here).

> distcp should place the file distcp_src_files in distributed cache
> ------------------------------------------------------------------
>
>                 Key: HADOOP-6072
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6072
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: tools/distcp
>    Affects Versions: 0.21.0
>            Reporter: Ravi Gummadi
>             Fix For: 0.21.0
>
>
> When large number of files are being copied by distcp, accessing distcp_src_files seems
to be an issue, as all map tasks would be accessing this file. The error message seen is:
> 09/06/16 10:13:16 INFO mapred.JobClient: Task Id : attempt_200906040559_0110_m_003348_0,
Status : FAILED
> java.io.IOException: Could not obtain block: blk_-4229860619941366534_1500174
> file=/mapredsystem/hadoop/mapredsystem/distcp_7fiyvq/_distcp_src_files
>         at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.chooseDataNode(DFSClient.java:1757)
>         at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.blockSeekTo(DFSClient.java:1585)
>         at org.apache.hadoop.hdfs.DFSClient$DFSInputStream.read(DFSClient.java:1712)
>         at java.io.DataInputStream.readFully(DataInputStream.java:178)
>         at java.io.DataInputStream.readFully(DataInputStream.java:152)
>         at org.apache.hadoop.io.SequenceFile$Reader.init(SequenceFile.java:1450)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1428)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1417)
>         at org.apache.hadoop.io.SequenceFile$Reader.<init>(SequenceFile.java:1412)
>         at org.apache.hadoop.mapred.SequenceFileRecordReader.<init>(SequenceFileRecordReader.java:43)
>         at org.apache.hadoop.tools.DistCp$CopyInputFormat.getRecordReader(DistCp.java:299)
>         at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:336)
>         at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
>         at org.apache.hadoop.mapred.Child.main(Child.java:170)
> This could be because of HADOOP-6038 and/or HADOOP-4681.
> If distcp places this special file distcp_src_files in distributed cache, that could
solve the problem.

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