It has been in the Solaris version of zfs for a long time as well. This came a few years after the Oracle-imposed fork.

https://blogs.oracle.com/solaris/post/reflink3c-what-is-it-w...

reflink is implemented on Oracle Solaris; how it works internally I don't think Oracle has ever commented, since implementing post-facto dedup on a filesystem that assumes data once written is unmodified is a bit Spicy(tm) - you can look at the BRT talk from the OpenZFS dev summit or other talks about it at the leadership meetings to get some idea why it's so exciting.

implying that zfs is not a commercial filesystem made me chuckle