I've run ZFS on OpenSolaris, Illumos, and then Linux, continuously since 2009. (However, I have no experience of using it on FreeBSD.) I've always regarded FreeNAS's RAM recommendations as being weirdly high. The "1GB per terabyte" thing dates from when ZFS systems were being pushed as enterprise workgroup file servers (i.e. maybe hundreds of users), with all storage on spinning rust. I wager that a great many FreeNAS systems are seeing approximately one user, primarily accessing large media files which mostly don't benefit that much from caching. Though of course, if you really are using FreeNAS in a more demanding role, then the hardware should reflect that.

I believe it did used to be the case that the FreeBSD port of ZFS had an issue where ZFS wouldn't always back off its RAM use fast enough, if the system came under memory presure, so it was at the time prudent to ensure that FreeBSD ZFS machines never got into low-memory situations, but I haven't heard anybody complain about that in practice in some years, so presumably it's fixed now, but FreeNAS's RAM recommendations have not been relaxed.

FreeNAS is famous for recommending the upper tier of ZFS requirements.

You can, and many people often do, run ZFS on a fraction of what FreeNAS recommends. In fact I’ve had ZFS running on a 32bot processor with 4GB RAM before. I’ve also had it running on 64bit with 2GB available RAM and it ran just fine for personal use.

However in fairness to FreeNAS, they are following best practices as defined by Sun/Oracle. It’s just those bear practices are defined with enterprise in mind.

I ran a ZFS file server and VM host for years with 6GB of RAM. It'll work fine, just don't use dedup.

With drive capacities topping 10TB and dedup being what it is, it's not too far of a stretch?

Those specs would be without dedup enabled (which generally isn’t in ZFS because it’s quite an expensive feature).

Yeah 8GB seems excessive. My Synology has 2GB of RAM and does just fine. I run multiple docker containers on it aswell.

That’s likely because Synology doesn’t offer ZFS or anything near the same capabilities.

synology uses btrfs which provides almost the same capabilities, but ZFS usually eats all the RAM it gets as some sort of cache to speed up disk operations.

You are thinking of the ARC. By default, ZFS will allocate what is available but you can constrain it as you like.