I'm trying to figure out how if rsyncing files is atomic. I couldn't find any confirmation about it. Due to rsync being able to send only deltas, I was under the impression that it also updates only parts of the live files.

On the other hang DJB recommends rsync for synchronising live .cdb files and I've found this post ( http://lists.samba.org/archive/rsync/2002-April/002210.html ) which both would imply that the new file is created, then moved over to the proper location.

Can someone point me at an official source confirming one or the other?

Rsync creates a new temporary file which will contain blocks from the local (receiving) copy and whatever blocks it requires from the sending copy. When done the temporary file is moved into place replacing the original.

Full details are here;

http://rsync.samba.org/how-rsync-works.html