tomscut edited a comment on pull request #3366:
URL: https://github.com/apache/hadoop/pull/3366#issuecomment-916639192


   @aajisaka @ayushtkn @ferhui @Hexiaoqiao Could you please review the code? 
Thanks a lot.
   
   The standard deviation is calculated by referring to 
FSNamesystem#getNodeUsage. The key change is 
FSNamesystem#getBlockPoolUsedPercentStdDev, :
   ```
     public float getBlockPoolUsedPercentStdDev(StorageReport[] storageReports) 
{
       ArrayList<Float> usagePercentList = new ArrayList<>();
       float totalUsagePercent = 0.0f;
       float dev = 0.0f;
   
       if (storageReports.length == 0) {
         return dev;
       }
   
       for (StorageReport s : storageReports) {
         usagePercentList.add(s.getBlockPoolUsagePercent());
         totalUsagePercent += s.getBlockPoolUsagePercent();
       }
   
       totalUsagePercent /= storageReports.length;
       Collections.sort(usagePercentList);
       for (Float usagePercent : usagePercentList) {
         dev += (usagePercent - totalUsagePercent)
             * (usagePercent - totalUsagePercent);
       }
       dev = (float) Math.sqrt(dev / usagePercentList.size());
       return dev;
     }
   ```


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