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https://issues.apache.org/jira/browse/HDFS-12077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16078867#comment-16078867
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Tsz Wo Nicholas Sze commented on HDFS-12077:
--------------------------------------------
Thanks for filing this JIRA. The idea sounds very useful when the disk usage
is high.
We also need to take care the case when the disk usage is low. For example,
- Percentage based
||DataNode ||Total Capacity || Used ||Remaining||utilization||
|DN1 | 20T | 2T | 18T |10%|
|DN2 | 100T | 10T | 90T |10%|
- Remaining space based
||DataNode ||Total Capacity || Used ||Remaining||utilization||
|DN1 | 20T | 0T | 20T | 0%|
|DN2 | 100T | 12T | 88T |12%|
As shown above, the remaining space based policy will keep the small datanodes
empty.
> 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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