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https://issues.apache.org/jira/browse/HADOOP-2559?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12558459#action_12558459
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eric baldeschwieler commented on HADOOP-2559:
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A further issue is that as discussed on the list, we get very uneven
distribution of data on the cluster when you have a small number of clients
writing a lot of data.
A preference for one copy on the same rack unless that rack is substantially
more full than most does make sense, but a preference for the same node seems
problematic.
Likewise, the choice of putting two blocks on the source rack seems to lead to
a lot of imbalance. We could get the same bandwidth reduction by putting 2
copies on the second rack if it has more free space than the source rack. We
could also choose the not allow two copies on a rack in the standard 3 replica
case, but that is a separable issue.
> DFS should place one replica per rack
> -------------------------------------
>
> Key: HADOOP-2559
> URL: https://issues.apache.org/jira/browse/HADOOP-2559
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Reporter: Runping Qi
>
> Currently, when writing out a block, dfs will place one copy to a local data
> node, one copy to a rack local node
> and another one to a remote node. This leads to a number of undesired
> properties:
> 1. The block will be rack-local to two tacks instead of three, reducing the
> advantage of rack locality based scheduling by 1/3.
> 2. The Blocks of a file (especiallya large file) are unevenly distributed
> over the nodes: One third will be on the local node, and two thirds on the
> nodes on the same rack. This may make some nodes full much faster than
> others,
> increasing the need of rebalancing. Furthermore, this also make some nodes
> become "hot spots" if those big
> files are popular and accessed by many applications.
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