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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-1332) When unable to place replicas, BlockPlacementPolicy should log reasons nodes were excluded
Date Wed, 11 May 2011 22:19:47 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1332?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13032123#comment-13032123

Todd Lipcon commented on HDFS-1332:

I think there's at least one more case worth adding "explanation" to. In testing this, I tried
creating a file when there were no datanodes running:

java.io.IOException: File /user/todd/x2 could only be replicated to 0 nodes, instead of 1

This is another case that new users run into a lot. It would be great to change the code in
FSNamesystem which throws this exception to check the total number of available nodes in the
system, and if it's < minReplication, include that in the message. Perhaps something like:

"File /foo/bar could only be replicated to 0 nodes, instead of 1, because there are 0 datanodes

Also, I tried testing this code by putting a file with a very large blocksize and my DN configured
on a small partition. I got the following NPE:

java.io.IOException: java.lang.NullPointerException
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.throwNotEnoughReplicasExceptionWithReason(BlockPlacementPolicyDefault.java:324)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseRandom(BlockPlacementPolicyDefault.java:367)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseLocalRack(BlockPlacementPolicyDefault.java:262)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseLocalNode(BlockPlacementPolicyDefault.java:236)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:171)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:139)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:89)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicy.chooseTarget(BlockPlacementPolicy.java:78)
        at org.apache.hadoop.hdfs.server.namenode.BlockPlacementPolicy.chooseTarget(BlockPlacementPolicy.java:234)
        at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:1637)
        at org.apache.hadoop.hdfs.server.namenode.NameNode.addBlock(NameNode.java:841)

> When unable to place replicas, BlockPlacementPolicy should log reasons nodes were excluded
> ------------------------------------------------------------------------------------------
>                 Key: HDFS-1332
>                 URL: https://issues.apache.org/jira/browse/HDFS-1332
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: Todd Lipcon
>            Assignee: Ted Yu
>            Priority: Minor
>              Labels: newbie
>         Attachments: HDFS-1332.patch
> Whenever the block placement policy determines that a node is not a "good target" it
could add the reason for exclusion to a list, and then when we log "Not able to place enough
replicas" we could say why each node was refused. This would help new users who are having
issues on pseudo-distributed (eg because their data dir is on /tmp and /tmp is full). Right
now it's very difficult to figure out the issue.

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