quick test program to assess disk write performance with and without fdatasync

test-disk-write-performance.c

/*
  write performance test

  tests disk write performance with and without fdatasync
  will write a configurable amount of 1MB data blocks to disk
  and then optionally call fdatasync after each
    
  usage:
  
  compile with 

      gcc -O3 -Wall -Wextra -std=c11 test-write-performance.c 

  and then run with

      time ./a.out /tmp/out 1000 0

  or 

      time ./a.out /tmp/out 1000 1
      
  The invocation parameters are:
  
  - output file (make sure to use an outfile on the disk of interest)
  - number of 1MB blocks to write
  - use fdatasync (0 = off, 1 = on)
*/

#define _POSIX_C_SOURCE 200112L

#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>


#define MB (1024ULL * 1024ULL)

int main(int argc, char* argv[]) {
  if (argc < 4) {
    fprintf(stderr, "usage: %s outfile mb sync\n", argv[0]);
    exit(1);
  }

  char const* outfile = argv[1];
  int mb = atoi(argv[2]);
  if (mb < 0) {
    fprintf(stderr, "usage: %s outfile mb sync\n", argv[0]);
    exit(1);
  }

  int sync = atoi(argv[3]);
  if (sync != 0 && sync != 1) {
    fprintf(stderr, "usage: %s outfile mb sync\n", argv[0]);
    exit(1);
  }

  char block[MB]; // 1 mb;
  memset(&block[0], 0xa5, sizeof(block)); 

  FILE* fp = fopen(outfile, "w");
  if (fp == NULL) {
    fprintf(stderr, "error while opening file: %s\n", strerror(errno));
    exit(1);
  }

  int fd = fileno(fp);
  if (fd < 0) {
    fprintf(stderr, "error determining file descriptor\n");
    exit(1);
  }
    
  fprintf(stdout, "about to write %llu bytes to %s %s sync\n", (unsigned long long) (mb * MB), outfile, sync ? "with" : "without");

  int total = 0;
  for (int i = 0; i < mb; ++i) {
    size_t written = fwrite(&block[0], sizeof(block), 1, fp);

    if (ferror(fp) != 0) {
      fprintf(stderr, "file output error\n");
      exit(1);
    }

    if (sync) {
      if (fdatasync(fd) == -1) {
        fprintf(stderr, "error while syncing file: %s\n", strerror(errno));
        exit(1);
      }
    }
    
    total += written;
    fprintf(stdout, "written %llu bytes of %llu\n", (unsigned long long) (total * MB), (unsigned long long) (mb * MB));
  }

  fclose(fp);

  return 0;
}