FileStreamOptions: PreallocationSize vs SetLength

[msedi]

When the FileStreamOptions were introduced it also contained the PreallocationSize in order to a preallocation of the file if the final size is known already at the beginning. Before I have always used SetLength which worked fine. My question now is, if I can safely use PreallocationSize instead of using SetLength?

Regarding the docs it says:

The initial allocation size in bytes for the file. A positive value is effective only when a regular file is being created or overwritten (Create or CreateNew). Negative values are not allowed. In other cases (including the default 0 value), it's ignored. This value is a hint and is not a strong guarantee. It is not supported on Web Assembly (WASM) and FreeBSD (the value is ignored). For Windows, Linux and macOS we will try to preallocate the disk space to fill the requested allocation size. If that turns out to be impossible, the operation is going to throw an exception. The final file length (EOF) will be determined by the number of bytes written to the file.

which makes me a little nervous because of "The value is a hint" and the "not supported" because none of these limitations have been shown in the docs reagrding SetLength (https://learn.microsoft.com/en-us/dotnet/api/system.io.filestream.setlength?view=net-6.0).

Obviously SetLength, nowadays calls into RandomAccess.SetLength and then into libSystem.Native where I'm currently not able to look into how it is implemented.

So what is the suggestions today? Using SetLength or PreallocationSize.