[jira] [Commented] (HDFS-17061) EC: Let data blocks and parity blocks on DNs more balanced

["Stephen O'Donnell (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-17061?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17737802#comment-17737802 ] 

Stephen O'Donnell commented on HDFS-17061:
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On a large cluster with many datanodes, and approximately random datanode selection for the pipelines, would the cluster balance out naturally?

Have you seen a problem like this on a large cluster, where the parity and data blocks are not reasonably well balanced?

> EC: Let data blocks and parity blocks on DNs more balanced
> ----------------------------------------------------------
>
>                 Key: HDFS-17061
>                 URL: https://issues.apache.org/jira/browse/HDFS-17061
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: balancer & mover, erasure-coding, hdfs
>            Reporter: WangYuanben
>            Priority: Minor
>         Attachments: figure1, unbalanced traffic load on DNs.png, figure2, balanced traffic load on DNs.png
>
>
> When choosing DN for placing data block or parity block, the existing number of data block and parity block on datanode is not taken into consideration. This may lead to *uneven traffic load*.
> As shown in the figure 1, when reading block group A, B, C, D and E from five different EC files without any missing block, datanodes like DN1 and DN2 will have high traffic load. However, datanodes like DN3, DN4 and DN5 may have low or even no traffic load. 
>  !figure1, unbalanced traffic load on DNs.png|width=600,height=333! 
> +If we can let data blocks and parity blocks on DNs more balanced, the traffic load in cluster will be more balanced and the peak traffic load on DN will be reduced+. Here "balance" refers to the matching of the number of data blocks and parity blocks on DN with its EC policy. In the ideal state, each DN has a balanced traffic load just like what figure 2 shows. 
>  !figure2, balanced traffic load on DNs.png|width=600,height=333! 
> Then how to reduce this imbalance? I think it's related to EC policy and the ratio of data blocks to parity blocks on datanode. For RS-3-2-1024k, it's appropriate to let the ratio close to 3:2. 
> There are two solutions:
> 1.Improve the block placement policy.
> 2.Improve the Balancer.



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