Thanks Garm, using a dedicated dataset for TM backup is what I'm doing, so I'll set the quota of the dataset to 1TB, as you said there's about a 10% difference of the numbers between Freenas and MAC, as long as it's woking fine I'll ignore the numbers.
PS: I just suffered from a failed TM restore from Freenas 2 week ago, the macos crushed and not able to boot, I tried to restore the TM backup from Freenas and it said to restore I need to erase the disk, I had no choice but to click "accept" and after 10 mins of restoring... it failed. I tried so many times and searching internet while I was doing it, nothing worked. I had to re-install macos from scratch, and tried to fix the TM backup (the process is very complicated), fortunately I was able to get back all my user data but have to re-do everything I did on macos these 2 years. Not sure if it's because I'm running 10.15 Catalina beta. Now I'm using TM to backup macos only, and backup my data with Rsync - this way I can be sure all the data I need is backed up and even the TM restore failed next time I won't need to find the data back.
I believe it's not Freenas's fault but mac's. It always told me that backup has been completed successfully but obviously something is wrong with the backup database, Freenas does everything it needs to do perfectly, once the backup is fixed and mounted I can get all the files, but not able to restore it with Timemachine.