fs.c

// create 1 million files, final directory size ~4.1gb:
// for i in {1..1024} ; do mkdir -p junk/$i ; for j in {1..1024} ; do cp /etc/hostname junk/$i/$j ; done; done
/*
[20:23:18 k2!58 ~] gcc -O2 -oa a.c
[20:23:19 k2!59 ~] time ./a

real    0m13.000s
*/

#include <sys/types.h>
#include <assert.h>
#include <sys/uio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

char rbuf[1048576];
char ri, rj;

int rnd(void)
{
    if(++ri > sizeof rbuf) {
        ri = 0;
    }
    return rbuf[ri] & ((1 << 10) - 1);
}

int main(void)
{
    char buffer[4096];
    char path[256];
    unsigned long i = 9999999;
    int n;

    int randfd = open("/dev/urandom", O_RDONLY);
    assert(sizeof rbuf == read(randfd, rbuf, sizeof rbuf));

    while(i--) {
        sprintf(path, "junk/%d/%d", 1+rnd(), 1+rnd());
        int fd = open(path, O_RDONLY);
        if(fd == -1) {
            perror(path);
            return -1;
        }
        n = pread(fd, buffer, 1, 0);
        close(fd);
    }
    printf("%d\n", (int)n);
}