From Freebsd Mastery: ZFS Pg 135
"For a rough-and-dirty approximation, you can assume that 1 TB of deduplicated data uses about 5 GB of RAM. You can more closely approximate memory needs for your particular data by looking at your data pool and doing some math. We recommend always doing the math and computing how much RAM your data needs, then using the most pessimistic result. If the math gives you a number above 5 GB, use your math. If not, assume 5 GB per terabyte."
Otherwise I think this is like the myth that ZFS requires expensive ECC ram whereas ECC ram is recommended for any filesystem and zfs needs it no more nor less.
