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https://issues.apache.org/jira/browse/HDFS-8131?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16558894#comment-16558894
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Yongjun Zhang commented on HDFS-8131:
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I just read HDFS-4946, I found it doesn't exactly do what I meant by comment #3
above.
HDFS-4946 introduced a config to disable/enable preferLocalDN, if disabled, the
localDN will be skipped for all application.
Whereas when I wrote comment #3 above, I was thinking that when choosing the
first DN, we could apply the same fix done here in HDFS-8131, such that we can
choose either local or remote for the first DN, instead of always skipping the
local DN.
Welcome to comment on this thought.
> Implement a space balanced block placement policy
> -------------------------------------------------
>
> Key: HDFS-8131
> URL: https://issues.apache.org/jira/browse/HDFS-8131
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 3.0.0-alpha1
> Reporter: Liu Shaohui
> Assignee: Liu Shaohui
> Priority: Minor
> Labels: BlockPlacementPolicy
> Fix For: 2.8.0, 2.7.4, 3.0.0-alpha1
>
> Attachments: HDFS-8131-branch-2.7.patch, HDFS-8131-v1.diff,
> HDFS-8131-v2.diff, HDFS-8131-v3.diff, HDFS-8131.004.patch,
> HDFS-8131.005.patch, HDFS-8131.006.patch, balanced.png
>
>
> The default block placement policy will choose datanodes for new blocks
> randomly, which will result in unbalanced space used percent among datanodes
> after an cluster expansion. The old datanodes always are in high used percent
> of space and new added ones are in low percent.
> Through we can used the external balance tool to balance the space used rate,
> it will cost extra network IO and it's not easy to control the balance speed.
> An easy solution is to implement an balanced block placement policy which
> will choose low used percent datanodes for new blocks with a little high
> possibility. In a not long term, the used percent of datanodes will trend to
> be balanced.
> Suggestions and discussions are welcomed. Thanks
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