Why is this not working as a disk speed test? - c

Question

I wrote a very simple program in C to test the read/write speed of the storage device where the program is located. It could be a SSD, HDD or a usb stick. But I get very inconsistent results, which is weird because the program is very simple and straightforward.
When I run it on a usb 3.0 stick it gives values like 270 mb/s [write], and 2100 mb/s [read].
For a HDD it gives similar values.
And for the SSD, it gives similar read speeds, and around 300 mb/s write speed.
This is weird, because there isn't anything complicated in the code, and I am not optimizing it either. The speeds reported don't match with the normal speeds of these devices. Though, it could be that I am not really understanding how this works.
#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
Be aware that it creates a 100 mb file.