[jira] [Created] (HDFS-16949) Update ReadTransferRate to ReadTransferTimePerByte for effective percentile metrics

["Ravindra Dingankar (Jira)" <ji...@apache.org>]

Ravindra Dingankar created HDFS-16949:
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             Summary: Update ReadTransferRate to ReadTransferTimePerByte for effective percentile metrics
                 Key: HDFS-16949
                 URL: https://issues.apache.org/jira/browse/HDFS-16949
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: datanode
            Reporter: Ravindra Dingankar
            Assignee: Ravindra Dingankar
             Fix For: 3.4.0, 3.3.0


HDFS-16917 added ReadTransferRate quantiles to calculate the rate which data is read per unit of time.

With percentiles the values are sorted in ascending order and hence for the transfer rate p90 gives us the value where 90 percent rates are lower (worse), p99 gives us the value where 99 percent values are lower (worse).

Note that value(p90) < p(99) thus p99 is a better transfer rate as compared to p90.

However as the percentile increases the value should become worse in order to know how good our system is.

Hence instead of calculating the data read transfer rate, we should calculate it's inverse. We will instead calculate the time taken for a byte of data to be read. ( seconds / byte )

After this the p90 value will give us 90 percentage of total values where the time taken is less than value(p90), similarly for p99 and others.

Also p(90) < p(99) and here p(99) will become a worse value (taking more time each byte) as compared to p(90)



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