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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-4430) Namenode Web UI capacity report is inconsistent with Balancer
Date Thu, 16 Oct 2008 17:57:44 GMT
Namenode Web UI capacity report is inconsistent with Balancer

                 Key: HADOOP-4430
                 URL: https://issues.apache.org/jira/browse/HADOOP-4430
             Project: Hadoop Core
          Issue Type: Bug
    Affects Versions: 0.19.0
            Reporter: Suresh Srinivas
            Assignee: Suresh Srinivas
             Fix For: 0.19.0

Solution to 2816 changed
- Total Capacity definition from (the disk space of all data directories) to (the disk space
of all the data directories - the reserved space)
- We added a new element Present Capacity to the report. It is set to (Used Capacity + Remaining
- We changed the Used Percentage reported from (Used Capacity)/(Total Capacity) to (Used Capacity)/(Present
- All these changes are displayed on Namenode Web UI.

Balancer functionality
Balancer script is started with a threshold parameter. It tries to move the blocks from the
nodes that have Used % that is more than (Cluster average + threshold) to the nodes that have
less than (Cluster average - threshold). Essentially balancer gets all the datanodes used
% to with in (the Cluster average +/- threshold).

Inconsistencies due to the change in 2816
When MapReduce jobs are run, temporary files are generated. This eats away a lot of space
from Present Capacity. The difference between the Total Capacity and the Present Capacity
can be huge. Currently balancer computes Used Percentage based (Used Capacity)/(Total Capacity).
The Used % the balancer uses could be significantly different from Used % displayed on the
Namenode Web UI. When balancer is done balancing, the Namenode Used % might still appear unbalanced.

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