I don't understand that because it would make more sense it the RAM requirments were contingent on mow many user clients were logging onto the NAS at one time requesting data. In other words, I have heard of the 1GB per TB rule, but if you had an 8TB array and were using it for a home NAS streaming media with one user at a time and on a rare occasion maybe two, why wouldn't 4GB work without those issues. And to further the scenario, if you had the same at home NAS, 1-2 users, but it was a 16TB NAS, why would you need 16GB or RAM?
You are partially correct in that more clients would make more RAM more useful, but it also has to do with pool size and usage patterns. ZFS performance tends to increase as RAM increases, basically because of the increase in read and write cache.
Our 30TB (12 4TB in RAIDZ3 with a spare) filer here is running under ESXi on a 128GB platform, so experimenting a bit I noticed that write performance when it had 32GB of RAM was about 3X what it was if I gave it 6GB. No users. Just writing data at full speed locally.
So the problem with complex systems is that they are complex. We develop rules of thumb like the 1GB/1TB things as a one-size-kinda-fits-most but there are still modifiers and qualifiers.
We say min 8GB RAM these days. for example, because less has been observed to sometimes result in unmountable pools in crash/powerfail scenarios.
We say 1GB/1TB because that seems to result in good average use case performance.
We also tend to double (or more!) the RAM size for VM storage usage largely because ZFS is a CoW filesystem and therefore 512-byte sector-sized writes in the middle of a file wreak a bit of havoc. Other things like L2ARC are further modifiers.
But basically if you know what you are doing and don't mind low performance the only rule I would consider "mandatory" is the 8GB min. Even that can be mitigated if you understand it.
We develop these guidelines for the benefit of newcomers.
But 100 users attached and doing nothing is a lot less stressy on a NAS than a single user using an iSCSI extent for video editing work. So the number of users is less relevant than what the NAS is actually doing. So we mostly consider use case, actual activity, and pool size as important factors. The number of users isn't generally too useful in considering RAM sizing.
