This might seem like a silly question but I cannot seem to find the answer so...
When calculating the amount of memory needed for a system with say 6 x 6TB drives using Z2 do you use the total RAID space 36 or the total data space 24?
So is that 36G or 24G of memory just needed for the storage? I'm guessing 24
If anyones interested I used this site
https://jsfiddle.net/Biduleohm/paq5u7z5/1/embedded/result/
@anmnz is close to the mark.
I am the party who originally bumped the minimum required RAM for FreeNAS with ZFS to 8GB upon observation that lower amounts resulted in unstable systems. So I'll explain the history here - firsthand sources are the best. ;-)
Systems with 4GB or 6GB of RAM used to see instability and data loss issues with ZFS. These are scattered throughout the early days of the forum, users with AMD APU's that maxed out at 4GB RAM, etc. If you put a stupid-large amount of storage on a system with insufficient RAM (let's say 32TB on 8GB) we also used to see instability, defined as lockups or panic events. Because there's been a historical ZFS rule of thumb to try to provision 1GB of RAM per TB of disk, I created some more modern guidance (at the time, ~2012-2013) and played the wording on it to be deliberately vague.
So the question is ... Is it:
1) Actual consumed space on the pool?
2) Total available pool space?
3) Total raw disk space?
The rule as I wrote it was deliberately vague. Home hobbyists who are hoping to store more data on less RAM are allowed to read it permissively and should consider something between 1) and 2). Serious enterprise users who demand performance should be looking more towards 3). This is a rule of thumb. It isn't a hard requirement. Your system requires what it requires. But that's a useless answer to people trying to build a NAS. So some sort of sizing guidance was necessary.
As HDD sizes have ballooned and ZFS has matured, though, and as you get away from stupid-small 8GB RAM sizes, the rules soften up and there's also a huge component of "what sort of things are you doing" that play a huge role. If you try to put 64TB of HDD on an 8GB RAM system, I *guarantee* you will see poor performance and you may even experience instability, though such reports are rather uncommon these days, so perhaps that is no longer a concern. But the system will not have sufficient metadata caching ability to rapidly find new free disk space or rapidly access many files. But does that 64TB require 64GB RAM? Probably not. You might get away with 32GB or 24GB without noticeable impact. But if you are driving the system heavily, you *still* might need 128GB (or more!) of RAM to get decent performance.
It is fine to start smaller and add RAM if you are unhappy. So the best advice for you is to start with the lower amount of RAM, and make sure that you are not putting something dumb like 2GB modules into your system and filling up all the RAM slots ("slot stuffing").