Do you know the implications for wear on the drive (related to TBW [total bytes written] and write amplification) on a CoW / highly fragmented drive? I figure if they are planning on writing once, then moving the whole file to an empty space, that means ~2x the writes.

Btrfs … do online defrag and deferred scheduling of tasks for it to allow for EVENTUAL consistency as far as contiguous block forms go. The more free space you have, the sooner it will become cleaner.

Based on my research, this has to be requested by the user/OS. I don’t think Openmediavault (Debian derivative) enables auto defrag out of the box. If I set this up manually, I definitely didn’t enable it. You can run a defrag command online (while the system is running) or set the mount to do it automatically.