I have added this request to the tracker:
Issue 2111 – Continuous allocation of disk space for local files
You can vote for it there.

@cracki: Not yet. None else has asked for this yet.

Any news on this feature?

I've just checked something... SetEndOfFile on Win7 seems to be instantaneous too! The whole file appears allocated AND it doesn't need privileges.

It might be sensible to truncate a file to what's really been written.

Thanks for your suggestion. We will consider it.

Hi!

It would be awesome if WinSCP could use this API to tell Windows to allocate a single contiguous file extent. As is, downloaded files can become quite fragmented, e.g. if two files are downloaded in parallel.

The most benefit would be to large files.
https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfilevaliddata

Another API, SetEndOfFile, would null out the whole file, which would block the call until that is done. SetFileValidData has some security concerns that WinSCP would pay attention to, but it needs zero time to do its magic.