Now I need to read/write a large file(15G),I want to accelerate the speed of reading /writing file, I find that the map function of QFilecan do it,I try it as follows,but I find that speed is the same as the common read/write file,why?

Qt Code: 
  1. QFile objInputFile("test.sgy");   
  2.     if (!objInputFile.open(QIODevice::Unbuffered | QIODevice::ReadOnly))   
  3.     {  
  4.         return false;   
  5.     }   
  6.  
  7.     QFile objOutputFile("out.sgy");   
  8.     if (!objOutputFile.open(QIODevice::Unbuffered | QIODevice::WriteOnly | QIODevice::Truncate))   
  9.     { 
  10.         objInputFile.close();   
  11.         return false;   
  12.     }   
  13.  
  14.     uchar * pData = NULL;   
  15.     qint64 m_qint64FileOffset = 0;
  16.     qint64 m_qint64InputFileSize = objInputFile.size(); 
  17. 	qint64 m_unBlockSize=m_qint64InputFileSize;
  18.  
  19.     qint64 unTmpBlockSize = m_qint64InputFileSize/10;   
  20.  
  21.     while(m_qint64FileOffset < m_qint64InputFileSize)   
  22.     {   
  23.        if (m_qint64InputFileSize-m_qint64FileOffset <unTmpBlockSize)  		
  24.               unTmpBlockSize=m_qint64InputFileSize-m_qint64FileOffset;		
  25.         pData = objInputFile.map(m_qint64FileOffset,unTmpBlockSize);
  26.  
  27.         if (pData == NULL)   
  28.         {   
  29.             objInputFile.close();   
  30.             objOutputFile.close();   
  31.             return false;   
  32.         }   
  33.         if (objOutputFile.write((char*)pData,unTmpBlockSize) == -1)   
  34.         {   
  35.             objInputFile.close();   
  36.             objOutputFile.close();   
  37.             return false;   
  38.         }   
  39.  
  40.         m_qint64FileOffset += unTmpBlockSize;   
  41.         if (!objInputFile.unmap(pData))   
  42.         {   
  43.             objInputFile.close();   
  44.             objOutputFile.close();   
  45.             return false;   
  46.         }   
  47.     } 
  48.  
  49.     objInputFile.unmap(pData);   
  50.     objInputFile.close();   
  51.     objOutputFile.close();   
  52.     return true;
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