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https://issues.apache.org/jira/browse/HDFS-12077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16078870#comment-16078870
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Tsz Wo Nicholas Sze commented on HDFS-12077:
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Therefore, it seems good to have a remaining space threshold: when the
remaining space > threshold, use percentage based. When the remaining space <=
threshold, use remaining space based.
> Implement a remaining space based balancer policy
> -------------------------------------------------
>
> Key: HDFS-12077
> URL: https://issues.apache.org/jira/browse/HDFS-12077
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: balancer & mover
> Affects Versions: 2.6.0
> Reporter: liuyiyang
>
> Our cluster has DataNodes with 2T disk storage, as storage utilization of the
> cluster growing, we need to add new DataNodes to increse the capacity of our
> cluster. In order to make utilization of every DataNode be in relatively
> balanced state, usually we use HDFS balancer tool to balance our cluster
> every time we add new DataNodes.
> We have been facing an issue with heterogeneous disk capacity when using HDFS
> balancer tool. In production cluster, we often have to add new DataNodes with
> larger disk capacity than previous DNs. Since the original balancer is
> implemented to balance utilization of every DataNode, the balancer will make
> every DN's utilization and average utilization of the cluster be within a
> given threshold.
> For example, in a cluster with two DataNodes DN1 and DN2, DN1 has ten disks
> with 2T capacity, DN2 has ten disks with 10T capacity, the original balancer
> may make the cluster balanced in the following state:
> ||DataNode||Total Capacity||Used||Remaining|| utilization||
> |DN1 | 20T | 18T| 2T| 90%|
> |DN2 | 100T | 90T | 10T|90%|
> each DN has reached a 90% utilization, in such a case, DN1's capacibility to
> store new blocks is far less than DN2's. When DN1 is full, all of the new
> blocks will be written to DN2 and more MR tasks will be scheduled to DN2. As
> a result, DN2 is overloaded and we can not
> make full use of each DN's I/O capacity. In such a case, We wish the balancer
> could run based on remaining space of every DN. After balancing, every DN's
> remaining space could be balanced like the following state:
> ||DataNode ||Total Capacity || Used ||Remaining||utilization||
> |DN1 | 20T | 14T | 6T |70%|
> |DN2 | 100T | 94T | 6T |94%|
> In a cluster with balanced remaining space of DN's capacity, every DN will be
> utilized when writing new blocks to the cluster, on the other hand, every
> DN's I/O capacity can be utilized when running MR jobs.
> Please let me know what you guys think. I will attach a patch if necessary.
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