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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-698) When DFS client fails to read from a datanode, the failed datanode is not excluded from target reselection
Date Tue, 28 Nov 2006 07:26:23 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-698?page=comments#action_12453815 ] 
            
Hadoop QA commented on HADOOP-698:
----------------------------------

+1, http://issues.apache.org/jira/secure/attachment/12345881/datanode-exclude.patch applied
and successfully tested against trunk revision 479931

> When DFS client fails to read from a datanode, the failed datanode is not excluded from
target reselection
> ----------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-698
>                 URL: http://issues.apache.org/jira/browse/HADOOP-698
>             Project: Hadoop
>          Issue Type: Bug
>          Components: dfs
>            Reporter: Hairong Kuang
>         Assigned To: Milind Bhandarkar
>         Attachments: datanode-exclude.patch
>
>
> In the method read(byte buf[ ], int off, int len) of DFSInputStream, when read fails,
 it calls "blockSeekTo" to reselect a datanode. However, the failed datanode does not feed
back to blockSeekTo. The datanode selection algorithm works as follows:
> * If the machine that the client is running on has a local copy, return the local machine;
> * Otherwise, randomly pick up one location.
> When the failed data node info does not feed back to target reselection, this leads to
two flaws:
> 1. When a client fails to read from the local copy, for example, because of the checksum
error, the local machine will always be chosen in retries.
> 2. Random selection may still return the same failed node.

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