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From "Stu Hood (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1652) Rebalance data blocks when new data nodes added or data nodes become full
Date Thu, 26 Jul 2007 20:58:03 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1652?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12515859

Stu Hood commented on HADOOP-1652:

...among which we should favor data nodes that are on the same rack as the target data node.

This is a tradeoff though... one of the reasons for recording what racks blocks are on is
to prevent putting "all of your eggs in one basket" so to speak (See the middle paragraph
of http://lucene.apache.org/hadoop/hdfs_design.html#Data+Replication ).

> Rebalance data blocks when new data nodes added or data nodes become full
> -------------------------------------------------------------------------
>                 Key: HADOOP-1652
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1652
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>    Affects Versions: 0.13.0
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.15.0
> When a new data node joins hdfs cluster, it does not hold much data. So any map task
assigned to the machine most likely does not read local data, thus increasing the use of network
bandwidth. On the other hand, when some data nodes become full, new data blocks are placed
on only non-full data nodes, thus reducing their read parallelism. 
> This jira aims to find an approach to redistribute data blocks when imbalance occurs
in the cluster.  An solution should meet the following requirements:
> 1. It maintains data availablility guranteens in the sense that rebalancing does not
reduce the number of replicas that a block has or the number of racks that the block resides.
> 2. An adminstrator should be able to invoke and interrupt rebalancing from a command
> 3. Rebalancing should be throttled so that rebalancing does not cause a namenode to be
too busy to serve any incoming request or saturate the network.

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