Re: High throughput disk write: CreateFile/WriteFile?

Ulrich Eckhardt <>
Fri, 29 Jun 2007 15:04:43 +0200
Brandon wrote:

I have an application where I need to write text and binary (numeric)
data to a file at ~200 MB/s (8MB/40ms). I am currently using fprintf
and fwrite, respectively, but I'm not achieving the desired

Just wondering, but what throughput did you manage to achieve?

I've been looking into the Windows CreateFile based methods instead.

This usually reduces the overhead a bit, because those are the native APIs
while fopen/fprintf/fwrite are just wrappers around them. However, even the
latter don't have to be slow, it still depends on how and what you are

    char szTextToAppendToLog[1024]; // Temp char buffer
    char szFilePath[1024]; // Output file name
    char szTimeStamp[32]; // Time character string
    char szTemp[128]; // Temp character string

I'm always wondering if hackers think that by using powers of two their
programs will somehow magically work correctly... (SCNR)

  hWriteFile = CreateFile(
     szFilePath, // File path
     GENERIC_WRITE, // Open for write
     NULL, // Do not share
     NULL, // Default security
     CREATE_ALWAYS, // Overwrite existing files
     NULL); // No template

One thing here: FILE_FLAG_WRITE_THROUGH means that this function will not do
any buffering. If you have short bursts of data to write, this will impact
performance negatively.

  if (hWriteFile == INVALID_HANDLE_VALUE)
        "ERROR: Output file %s failed to open.",

You should throw an exception here, continuing here is just plain wrong.

  // Write ASCII header to file.
  sprintf_s(szTemp, sizeof(szTemp), "MyData: Date:%s\r\n",szTimeStamp);
     (DWORD) sizeof(szTemp),

Apart from the NULL-pointer for the number of written bytes, Here might be a
reason for performance problems. The problem here is that you are writing
short pieces of data but without intermediate buffering. Also, I'd suggest
not doing any C-style casts ("(DWORD) sizeof(szTemp)") because those bear
the danger of hiding errors.


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