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http://issues.apache.org/jira/browse/HADOOP-124?page=comments#action_12414118 ] 

Doug Cutting commented on HADOOP-124:
-------------------------------------

This adds a number of new public classes that I'm not certain should be public. 
 Should user code ever need to access a DataStorage, DatanodeID or 
DatanodeRegistration, or are these only used internally?  Also, several of 
these exceptions appear only to be used internally, but I'm not certain about 
all of them.  Would you object if I simply make all of these new classes 
package-private?  Then, if we need, we can reveal more later as needed.

> don't permit two datanodes to run from same dfs.data.dir
> --------------------------------------------------------
>
>          Key: HADOOP-124
>          URL: http://issues.apache.org/jira/browse/HADOOP-124
>      Project: Hadoop
>         Type: Bug

>   Components: dfs
>     Versions: 0.2
>  Environment: ~30 node cluster
>     Reporter: Bryan Pendleton
>     Assignee: Konstantin Shvachko
>     Priority: Critical
>      Fix For: 0.3
>  Attachments: DatanodeRegister.txt, Hadoop-124-v3.patch, Hadoop-124.patch
>
> DFS files are still rotting.
> I suspect that there's a problem with block accounting/detecting identical 
> hosts in the namenode. I have 30 physical nodes, with various numbers of 
> local disks, meaning that my current 'bin/hadoop dfs -report" shows 80 nodes 
> after a full restart. However, when I discovered the  problem (which resulted 
> in losing about 500gb worth of temporary data because of missing blocks in 
> some of the larger chunks) -report showed 96 nodes. I suspect somehow there 
> were extra datanodes running against the same paths, and that the namenode 
> was counting those as replicated instances, which then showed up 
> over-replicated, and one of them was told to delete its local block, leading 
> to the block actually getting lost.
> I will debug it more the next time the situation arises. This is at least the 
> 5th time I've had a large amount of file data "rot" in DFS since January.

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