Now for Windows, "Fast Allocate" do the same as "Sparse Files" while doing "File Allocation". There are already many discussions here:

https://forum.tixati.com/support/524
https://forum.tixati.com/support/3056
https://forum.tixati.com/support/5764
https://forum.tixati.com/support/6406
https://forum.tixati.com/support/7116


In general, why we need "Fast Allocate"? Because other two do not do well:

1. "Full Pre-write" is slow and write twice.
2. "Sparse Files" makes too much file fragments, bad to HDD. (SDD is OK)


I have tested "Fast Allocate" on Windows, it is easy to implement, via use APIs SetFilePointerEx and SetEndOfFile, without Admin authority.

Here is the test, use Python.


import ctypes
import os
import stat
import time

is_admin = ctypes.windll.shell32.IsUserAnAdmin()
print(f'This is{"" if is_admin else " not"} a elevated process.')

NULL = None
GENERIC_READ = 0x80000000
GENERIC_WRITE = 0x40000000
FILE_SHARE_READ = 0x00000001
FILE_SHARE_WRITE = 0x00000002
OPEN_ALWAYS = 4
FILE_ATTRIBUTE_NORMAL = 0x80
FILE_BEGIN = 0
INVALID_HANDLE_VALUE = -1

filepath = 'testfile'
filepath = os.path.abspath(filepath)
filesize = 100  # MB
offset = (1 << 20) * filesize
startn = time.time()

hFile = ctypes.windll.Kernel32.CreateFileW(
           filepath, GENERIC_READ | GENERIC_WRITE,
           FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_ALWAYS,
           FILE_ATTRIBUTE_NORMAL, NULL)
assert hFile != INVALID_HANDLE_VALUE
assert ctypes.windll.Kernel32.SetFilePointerEx(hFile, offset, NULL, FILE_BEGIN)
assert ctypes.windll.Kernel32.SetEndOfFile(hFile)
assert ctypes.windll.Kernel32.CloseHandle(hFile)

costn = (time.time() - startn) * 1e3
is_sparse = stat.FILE_ATTRIBUTE_SPARSE_FILE & os.stat(filepath).st_file_attributes

input(f'''
File "{filepath}" has been created.
   File Size: {filesize} MB
   Is Sparse: {bool(is_sparse)}
   Cost Time: {costn} ms

Press Enter to delete test file and exit...''')

os.remove(filepath)


It's not just Windows, Linux has the same problem.