As far as I could tell, FreeBSD ZFS does not support copy-on-write
using cp; the native cp does not seem to have an option for such
lightweight copies, and trying GNU cp with
--reflink errors out on
the ZFS system I tried with error message "cp: failed to clone
'example.bak' from 'example.log': Operation not supported".
A commenter mentions that Solaris cp has a
-z switch to do such
However, and I hope this answers your underlying question,
copy-on-write is used for filesystem snapshots: let's say you have
900GB used out of 1000GB available, nothing prevents you from making a
snapshot of that filesystem, the snapshot will not occupy 900GB; in
fact, it will initially not occupy any new data blocks at all.
After creating a snapshot of your original filesystem containing
example.log, you end up with two "copies": the read-only version in
the snapshot, and you live version in its original location. What
happens when the copy is modified, be it by appending or by being
altered in-place? That is where the magic happens: only those blocks
that are altered get copied and start using up space. It is not the
case that the entire file gets copied as soon as it gets altered.