Reserve disk space before writing a file for efficiency

Question

I have noticed a huge performance hit in one of my projects when logging is enabled for the first time. But when the log file limit is reached and the program starts writing to the beginning of the file again, the logging speed is much faster (about 50% faster). It's normal to set the log file size to hundreds of MBs.

Most download managers allocate dummy file with the required size before starting to download the file. This makes the writing more effecient because the whole chunk is allocated at once.

What is the best way to reserve disk space efficiently, by some fixed size, when my program starts for the first time?

Answer 1

Why can't you just create an empty log using any size you want?

Answer 2

wRAR is correct. Open a new file using your favourite library, then seek to the penultimate byte and write a 0 there. That should allocate all the required disk space.

Answer 3

void ReserveSpace(LONG spaceLow, LONG spaceHigh, HANDLE hFile)
{
    DWORD err = ::SetFilePointer(hFile, spaceLow, &spaceHigh, FILE_BEGIN);

    if (err == INVALID_SET_FILE_POINTER) {
        err = GetLastError();
        // handle error
    }
    if (!::SetEndOfFile(hFile)) {
        err = GetLastError();
        // handle error
    }
    err = ::SetFilePointer(hFile, 0, 0, FILE_BEGIN); // reset
}

Answer 4

Here's a simple function that will work for files of any size:

void SetFileSize(HANDLE hFile, LARGE_INTEGER size)
{
    SetFilePointer(hFile, size, NULL, FILE_BEGIN);
    SetEndOfFile(hFile);
}