#include <ctime>
#include <ios>
#include <iostream>
#include <vector>
#include <fstream>
#include <assert.h>

int main()
{
  clock_t start = clock();

  // 512M
  std::fstream fa("d:\\a.txt", std::ios::binary|std::ios::out|std::ios::in);
  if (!fa.is_open()) {
    std::cout << "Failed to open"<<std::endl;
    return 1;
  }

  // 130M
  std::streamsize sa = fa.seekg(0, std::ios::end).tellg();
  std::cout << sa << std::endl;
  fa.close();

  std::fstream fb("d:\\b.txt", std::ios::binary|std::ios::in);
  if (!fb.is_open()) {
    std::cout << "Failed to open" << std::endl;
    fa.close();
    return 1;
  }
  std::streamsize sb = fb.seekg(0, std::ios::end).tellg();
  std::cout << sb << std::endl;

  std::streamsize begin = 1024 * 1024 * 50; // 50M

  std::vector<char> buf((unsigned int)sb);
  fb.seekg(0, std::ios::beg).read(&buf[0], sb);
  fa.seekp(begin, std::ios::beg).write(&buf[0], sa);

  clock_t end = clock();
  std::cout << "time : " << ((double)end - start) / CLOCKS_PER_SEC << "s\n";

  fa.close();
  fb.close();
  return 0;
}

测试了下512M文件a 偏移50M 位置 更新为130M文件b 大约0.6秒
你可以测试下更大的文件,如果内存不够的话可以分块来操作