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From "liuyiyang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-12077) Implement a remaining space based balancer policy
Date Sun, 02 Jul 2017 14:16:00 GMT
liuyiyang created HDFS-12077:

             Summary: 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 an 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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