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GH-142407: Doc: Clarify shutil.copyfile() does not use CopyFile2 on Windows #144354

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GH-142407: Doc: Clarify shutil.copyfile() does not use CopyFile2 on Windows #144354

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14 changes: 11 additions & 3 deletions Doc/library/shutil.rst
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Expand Up @@ -538,9 +538,17 @@ On Linux :func:`os.copy_file_range` or :func:`os.sendfile` is used.

On Solaris :func:`os.sendfile` is used.

On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
instead of 64 KiB) and a :func:`memoryview`-based variant of
:func:`shutil.copyfileobj` is used.
On Windows, :func:`shutil.copy`, :func:`shutil.copy2`, :func:`shutil.copytree`,
and :func:`shutil.move` use the native ``CopyFile2`` API (available on Windows 8
and later) when possible. This performs the copy entirely in kernel space,
supports copy-on-write, and avoids copying data through userspace buffers,
resulting in significantly better performance—especially for large files or
files on network shares.

In contrast, :func:`shutil.copyfile` does **not** use ``CopyFile2``; it still
performs a userspace read/write loop (albeit with a larger 1 MiB buffer since
Python 3.8). Therefore, for best performance on Windows, prefer :func:`copy2`
over :func:`copyfile` when preserving metadata (like timestamps) is acceptable.

If the fast-copy operation fails and no data was written in the destination
file then shutil will silently fallback on using less efficient
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