zhouyingchao created HDFS-8045:
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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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