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https://issues.apache.org/jira/browse/HDFS-8131?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15981732#comment-15981732
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Ruslan Dautkhanov commented on HDFS-8131:
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Thanks for this great improvement!
When using AvailableSpaceBlockPlacementPolicy, the default below logic does not
work anymore?
{quote}
1. Place the first replica somewhere – either a random rack and node (if the
HDFS client is outside the hadoop cluster) or on the local node (if the HDFS
client is running on a node inside the cluster).
2. The second replica is written to a different rack from the first, chosen at
random.
3. The third replica is written to the same rack as the second replica, but on
a different node.
4. If there are more replicas – spread them across the rest of the racks.
{quote}
What is this logic now? When it comes to rackawareness and such?
Is it by pure available space and rack awareness logic doesn't kick in?
> 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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