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https://issues.apache.org/jira/browse/HDFS-13279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16400065#comment-16400065
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Tao Jie commented on HDFS-13279:
--------------------------------
It seems not easy to make probability absolutely equal for each node, but we
can add a modifier to those nodes in smaller rack and avoid them from being
written too frequently.
Any thought?
> Datanodes usage is imbalanced if number of nodes per rack is not equal
> ----------------------------------------------------------------------
>
> Key: HDFS-13279
> URL: https://issues.apache.org/jira/browse/HDFS-13279
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 2.8.3, 3.0.0
> Reporter: Tao Jie
> Priority: Major
>
> In a Hadoop cluster, number of nodes on a rack could be different. For
> example, we have 50 Datanodes in all and 15 datanodes per rack, it would
> remain 5 nodes on the last rack. In this situation, we find that storage
> usage on the last 5 nodes would be much higher than other nodes.
> With the default blockplacement policy, for each block, the first
> replication has the same probability to write to each datanode, but the
> probability for the 2nd/3rd replication to write to the last 5 nodes would
> much higher than to other nodes.
> Consider we write 50 blocks to such 50 datanodes. The first rep of 100 block
> would distirbuted to 50 node equally. The 2rd rep of blocks which the 1st rep
> is on rack1(15 reps) would send equally to other 35 nodes and each nodes
> receive 0.428 rep. So does blocks on rack2 and rack3. As a result, node on
> rack4(5 nodes) would receive 1.29 replications in all, while other node would
> receive 0.97 reps.
> ||-||Rack1(15 nodes)||Rack2(15 nodes)||Rack3(15 nodes)||Rack4(5 nodes)||
> |From rack1|-|15/35=0.43|0.43|0.43|
> |From rack2|0.43|-|0.43|0.43|
> |From rack3|0.43|0.43|-|0.43|
> |From rack4|5/45=0.11|0.11|0.11|-|
> |Total|0.97|0.97|0.97|1.29|
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