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https://issues.apache.org/jira/browse/HDFS-8131?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15988564#comment-15988564
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maobaolong commented on HDFS-8131:
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[~liushaohui]
Hello, Thanks for this great improvement!
I have an question about the config key
DFS_NAMENODE_AVAILABLE_SPACE_BLOCK_PLACEMENT_POLICY_BALANCED_SPACE_PREFERENCE_FRACTION_DEFAULT.
What is the situation when this key is set between 0.5 and 1.0, i think it is
better to set to 1.0 always, Is that true? There must be something i don't
understand well, Please point it, thank you advance.
> 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, 3.0.0-alpha1
>
> Attachments: balanced.png, HDFS-8131.004.patch, HDFS-8131.005.patch,
> HDFS-8131.006.patch, HDFS-8131-v1.diff, HDFS-8131-v2.diff, HDFS-8131-v3.diff
>
>
> 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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