zfs need ram only if deduplication enabled. i use FreeNAS with 2GB ram long time, and it stable.
I would say this is not entirely correct. If you read best practices for ZFS, you will often come across the mantra that one should have 1Gb of memory for each Tb of data and that 8Gb is a practical minimum. It is not just a FreeNAS bias: try searching forums outside of FreeNAS and you will find many comments to this effect. Certainly, under certain situations one can get by with less: one or two small disks without redundancy, perhaps a small mirror, but there will be significant safety and performance compromises. I actually tried FreeNAS 9 on an old 2Gb setup to see what would happen - it was not pretty and I wouldn't waste my time with it.
Back in the old FreeNAS 7 days, I ran a system with 2Gb RAM, using ext2 on a single 2Tb WD green drive. It worked just fine, but it was a file repository - nothing more. With FreeNAS 8 (the first version that supported plugins) I had to increase the memory to 4Gb and remained on ext2 filesystem. When I decided to update to FreeNAS 9, I also upgraded hardware because I wanted to use ZFS. I have had no problems except those I created myself by doing something stupid.
Last thought: you can look at memory performance in the Webgui to see how much memory is being used by your system. For my current home system with a 2x2Tb mirror, the system has allocated about 6.5Gb memory. For a large Raidz or Raidz2 array, it would allocate much more.
OK, one more thought: If you look at the signatures of the frequent posters on the NAS4free site, you will commonly see configurations that have more than 8Gb of RAM. Just because they make the claim that you can run the system in 1Gb of memory doesn't mean that you would want to.
