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From "Tsz Wo Nicholas Sze (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-12077) Implement a remaining space based balancer policy
Date Sat, 08 Jul 2017 00:44:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16078867#comment-16078867
] 

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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