Microbenchmark that executes 1000 writes of the requested size (up to 8KB). This is a slight tweak on bench.c

bench2.c

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

const char* buf = "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";

int main(int argc, const char** argv) {
    int len = 1;
    if (argc > 1)
        len = atoi(argv[1]);
    printf("Writing %d bytes at a time\n", len);
    int fd = open("bench.dat", O_WRONLY|O_CREAT|O_TRUNC, 0666);
    if (fd < 0) {
        fprintf(stderr, "open failed: %s\n", strerror(errno));
        exit(1);
    }
    int r = posix_fallocate(fd, 0, 1000 * 1024);
    if (r != 0) {
        fprintf(stderr, "fallocate failed: %s\n", strerror(r));
        exit(1);
    }
    r = fsync(fd);
    if (r != 0) {
        fprintf(stderr, "fsync failed: %s\n", strerror(errno));
        exit(1);
    }
    int i;
    for (i = 0; i < 1000; ++i) {
        r = write(fd, buf, len);
        if (r != len) {
            fprintf(stderr, "write failed: %s\n", strerror(errno));
            exit(1);
        }
        r = fdatasync(fd);
        if (r != 0) {
            fprintf(stderr, "fdatasync failed: %s\n", strerror(errno));
            exit(1);
        }
    }
    return 0;
}

Bug: forgot to adjust the fallocate call. Should be 1000 * len.