c - 为什么这不能作为磁盘速度测试？

Question

我用C写了一个非常简单的程序来测试程序所在存储设备的读写速度。它可以是 SSD、HDD 或 USB 记忆棒。但是我得到的结果非常不一致，这很奇怪，因为程序非常简单明了。

当我在 USB 3.0 记忆棒上运行它时，它会给出 270 mb/s [写入] 和 2100 mb/s [读取] 之类的值。对于 HDD，它给出了类似的值。对于 SSD，它提供类似的读取速度和大约 300 mb/s 的写入速度。

这很奇怪，因为代码中没有任何复杂的东西，我也没有优化它。报告的速度与这些设备的正常速度不匹配。不过，可能是我并不真正了解它是如何工作的。

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>
#include <math.h>

const unsigned int N = 25000000;                         /// Number of floats to be written

int main(){
    double time0, time1, time2;
    unsigned int i, size = N*sizeof(float);              /// Size of the file to be written/read in bytes
    FILE *pfin, *pfout;

    float *array_write, *array_read, sum, delta_write, delta_read;

    array_write = (float *) malloc(N*sizeof(float));    /// Array to be written to a file
    array_read  = (float *) malloc(N*sizeof(float));    /// Array to be read


    for(i = 0; i < N; i++)
        array_write[i] = i*1.f/N;                        /// Filling in array with some values

    time0 = omp_get_wtime();
    pfout = fopen("test.dat", "wb");
    fwrite(array_write, N*sizeof(float), 1, pfout);
    fclose(pfout);

    time1 = omp_get_wtime();
    pfin  = fopen("test.dat", "rb");
    fread(array_read, N*sizeof(float), 1, pfin);
    fclose(pfin);

    time2 = omp_get_wtime();

    sum = 0.f;
    for(i = 0; i < N; i++)
        sum += fabsf(array_read[i] - array_write[i]);  /// Simple test to check whether it read properly or not

    delta_write = time1 - time0;
    delta_read  = time2 - time1;


    printf("delta1 = %f, delta2 = %f, size = %f Gb, diff = %f\n", delta_write, delta_read, size/1000000000.f, sum);


    printf("Speed: \n Write: %f [Mb/s]\n Read: %f [Mb/s]\n", size/1000000.f/delta_write, size/1000000.f/delta_read);
    free(array_read);
    free(array_write);
}


//// compile with gcc program.c -lgomp -lm -O0 -o program.x

请注意，它会创建一个 100 mb 的文件。

标签： cbinaryfwritefreaddisk