Hi I'm in the middle of building a Freenas box and at the point of buying the drives which will either be WD or HGST. I have 32 gigs of ram so can have 24 gigs of storage so is it best to have eight 3TB drives or six 4TB drives. I was planning on running it with 1 disk as parity if this makes a difference.
Thanks in advance
Personally I'd go with 8x 3TB and have 2 drives parity. Rebuilding an array after a drive failure is heavy on the other drives, so you may want to account for one of those failing too, in which case a RAIDZ1 array will break. Also, @Logan can testify how easy it is to lose 2 drives, and not all of us have direct access to a sorcerer that can recover the data.
The RAM requirement is just a guideline, not a law. You can push it a little if you want.
In fact I'm running a 32TB NAS with RAIDZ2 (parity and overhead means I have about 20TB usable) on 16GB of RAM and I haven't encountered any issues yet. RAM usage is high, but that would still be the case if I were to double it.
The only bottleneck I encountered so far is the speed of my home network (gigabit). No matter what I do, it's reading and writing at 120-125MB/s.
I would go with HGST due to Black Blaze's results. Also as far as I know larger capacity HDD's are less reliable than smaller capacity HDD's. So I would say go ahead and pick up the eight 3TB drives but either way you can't really go wrong.
With that much space you are probably going to want to run at least a Z2 with 2 disc parity. If you lose a drive, there is a probability that you would lose a second drive while resilvering. You will lose the data if this happens unless you have an off site backup. The FreeNAS documentation doesn't recommend using RaidZ1 with drives over 1TB in size because the probability of an error or failed drive increases with the size of the drives in the pool. The larges pool in my box is a 6 x 3TB array yielding 12TB's usuable space after formatting as RaidZ2.