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From "zhouyingchao (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-8045) Incorrect calculation of NonDfsUsed and Remaining
Date Thu, 02 Apr 2015 13:29:42 GMT
zhouyingchao created HDFS-8045:

             Summary: Incorrect calculation of NonDfsUsed and Remaining
                 Key: HDFS-8045
                 URL: https://issues.apache.org/jira/browse/HDFS-8045
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: zhouyingchao
            Assignee: zhouyingchao
         Attachments: HDFS-8045-001.patch

After reserve some space via the param "dfs.datanode.du.reserved", we noticed that the namenode
usually report NonDfsUsed of Datanodes as 0 even if we actually write some data to the volume.
After some investigation, we think there is an issue in the calculation of FsVolumeImpl.getAvailable
- following is the explaination.

For a volume, let's use Raw to represent raw capacity, DfsUsed to represent space consumed
by hdfs blocks, Reserved to represent reservation through "dfs.datanode.du.reserved", RbwReserved
to represent space reservation for rbw blocks, NDfsUsed to represent real value of NonDfsUsed(which
will include non-hdfs files and meta data consumed by local filesystem).
In current implementation, for a volume, available space will be actually calculated as  min{Raw
- Reserved - DfsUsed -RbwReserved,  Raw - DfsUsed - NDfsUsed }. 
Later on, Namenode will calculate NonDfsUsed of the volume as "Raw - Reserved - DfsUsed -
min{Raw - Reserved - DfsUsed - RbwReserved, Raw - DfsUsed - NDfsUsed}".

Given the calculation, finally we will have -
if "Reserved + RbwReserved > NDfsUsed", then the calculated NonDfsUsed will be RbwReserved.
Otherwise if "Reserved + RbwReserved < NDfsUsed", then the calculated NonDfsUsed would
be "NDfsUsed - Reserved". Either way it is far from a correct value.

After investigation the implementation, we believe the Reserved and RbwReserved should be
subtract from available in getAvailable since they are actually not available to hdfs in any
way.  I'll post a patch soon.

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