Uh, thanks for the personal attack? I am aware that cloning exists, and I very explicitly allowed for the use of such a mechanism to change the conclusion in both of my comments. My trouble was that I wasn't sure how much filesystem-cloning is actually in use in relevant contexts. Does POSIX have some sort of "copyfile()" system call nowadays? Last I knew (outdated, I'm sure), the cp command for example seemed to just read() blocks into a buffer and write() them out again. I'm not sure how the filesystem layer would detect this as a clone without a lookup. I was quoting and basing my assumptions on the article:

> The downside is that every single write and free operation requires a lookup and a then a write to the dedup table, regardless of whether or not the write or free proper was actually done by the pool.

Which, if universally true, is very much different from what a hypervisor could do instead, and I've detailed the potential differences. But if a hypervisor does use some sort of clone system call instead, that can indeed shift the same approach into the fs layer, and my genuine question is whether it does.