If I start xcp normally, it shows as if the file were copied instantly, while in reality it is copied to cache (/proc/meminfo shows Dirty: as big as the file), with further actual copying happening outside xcp's control (i.e. progress bar).

If I start xcp in a memory-limited cgroup, it copies the file gradually (dirty bytes does not exceed the limit I set for it, I see destination file size gradually increasing), but the progress bar is stuck at 0% until the file finishes growing to its normal size. After that it starts actually copying the file (with normal progress bar). The data is written to vfat USB drive twice: first zero bytes, then actual content.

Shall xcp detect that it is copying the data to a USB drive and activate some O_DIRECT mode so that copy does not touch the cache and is easily abortable by user?