Preallocate disk space for a file in Python without changing its size

I did a bit of googling and found this lovely article with some C code to do exactly what you're asking on Windows. Here's that C code translated to ctypes (written for readability):

    import ctypes
    import msvcrt
    # https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfileinformationbyhandle
    set_file_information = ctypes.windll.kernel32.SetFileInformationByHandle

    class AllocationInfo(ctypes.Structure):
        _fields_ = [('AllocationSize', ctypes.c_longlong)]
    
    def allocate(file, length):
        """Tell the filesystem to preallocate `length` bytes on disk for the specified `file` without increasing the
        file's length.
        In other words, advise the filesystem that you intend to write at least `length` bytes to the file.
        """
        allocation_info = AllocationInfo(length)
        retval = set_file_information(ctypes.c_long(msvcrt.get_osfhandle(file.fileno())),
                                      ctypes.c_long(5),  # constant for FileAllocationInfo in the FILE_INFO_BY_HANDLE_CLASS enum
                                      ctypes.pointer(allocation_info),
                                      ctypes.sizeof(allocation_info)
                                      )
        if retval != 1:
            raise OSError('SetFileInformationByHandle failed')

This will change the file's Size on disk: as shown in file explorer to the length you specify (plus a few kilobytes for metadata), but leave the Size: unchanged.

However, in the half hour I've spent googling, I've not found a way to do that on POSIX. fallocate() actually does the exact opposite of what you're after: it sets the file's apparent length to the length you give it, but allocates it as a sparse extent on the disk, so writing to multiple files simultaneously will still result in fragmentation. Ironic, isn't it, that Windows has a file management feature that POSIX lacks?

I'd love nothing more than to be proven wrong, but I don't think it's possible.

FILENAME = "somefile.bin"
SIZE = 4200000

with open(FILENAME, "wb") as file:
    file.seek(SIZE - 1)
    file.write(b"\0")

Advantages:

1. Portable across all platforms.
2. Very efficient if you'd be mmaping (memory-mapping) the files to perform writes on them (via MADV_SEQUENTIAL if sequential access is needed).