[jira] Commented: (HADOOP-2094) DFS should not use round robin policy in determing on which volume (file system partition) to allocate for the next block

["Runping Qi (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-2094?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12579724#action_12579724 ] 

Runping Qi commented on HADOOP-2094:
------------------------------------


Have we (H-2559) decided not to place the first block replica on the local node?


> DFS should not use round robin policy in determing on which volume (file system partition)  to allocate for the next block
> --------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-2094
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2094
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: Runping Qi
>            Assignee: dhruba borthakur
>             Fix For: 0.17.0
>
>         Attachments: randomDatanodePartition.patch
>
>
> When multiple file system partitions are configured for the data storage of a data node,
> it uses a strict round robin policy to decide which partition to use for writing the next block.
> This may result in anormaly cases in which the blocks of a file are not evenly distributed across 
> the partitions. For example, when we use distcp to copy files with each node have 4 mappers running concurrently, 
> those 4 mappers are writing to DFS at about the same rate. Thus, it is possible that the 4 mappers write out
> blocks interleavingly. If there are 4 file system partitions configured for the local data node, it is possible that each mapper will
> continue to write its blocks on to the same file system partition.
> A simple random placement policy will avoid such anormaly cases, and does not have any obvious drawbacks.
>  

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