First of all let me start with some advice I wish I would have known before I started. Never start a storage server product without the right amount of drives equal to the space and redundancy needed for you. Please see https://www.servethehome.com/raid-calculator/ to determine what works best for you.
Next lets take a look at some information Iam borrowing from http://www.zfsbuild.com/2010/05/26/zfs-raid-levels/ regarding what each raid level does. I take no credit for this part of the information please see above link.
I would not use this (personally) myself because one drive redundancy is not a good idea. If a drive dies it puts a lot more strain on the rest of array during rebuild and chances are higher another drive can die. If that were to happen all data in that pool would be lost
RAIDZ is very popular among many users because it gives you the best tradeoff of hardware failure protection vs useable storage. It is very similar to RAID5, but without the write-hole penalty that RAID5 encounters. The drawback is that when reading the checksum data, you are limited to basically the speed of one drive since the checksum data is spread across all drives in the zvol. This causes slowdowns when doing random reads of small chunks of data. It is very popular for storage archives where the data is written once and accessed infrequently. How To Create RAIDZ Zpool
This is the most commonly used level among most freenas user and the most suggested use drive. Keep in mind you loose to drives worth of space when making your decision.
RAIDZ2 is like RAID6. You get double parity to tolerate multiple disk failures. The performance is very similar to RAIDZ. How To Create RAIDZ2 Zpool
This is like RAIDZ and RAIDZ2, but with a third parity point. This allows you to tolerate 3 disk failures before losing data. Again, performance is very similar to RAIDZ and RAIDZ2.
Now that is out of the way. We can go over a few more things. For the love of all things holy please use a recommended raid/hhba card on the supported list. I would take a good look at https://forums.freenas.org/index.php?threads/hardware-recommendations-read-this-first.23069/
Please keep in mind the guide is somewhat dated, but should get point across. There is a very large argument in the freenas community about ECC ram and that is highly recommended with how zfs works. I don’t personally used it myself, but knowing this I have a fully backed up 1 to 1 mirror of all my data. I am going to skip this part for better part of not starting another war.
PLEASE PLEASE PLEASE PLEASE use good working hard drives and h ave the amount needed BEFORE jumping in. a BIG thing to keep in mind once you start and build a vdev you CANT CHANGE OR ADD to that pool. You can however create a brand new vdev keeping like drives recommended. See guides for more details on that.
I have been using freenas for better part of a year now. I think its a very rock solid storage platform. There is always pros and cons to each solution out there. This mights my needs perfectly and there is a good amount of plugins as well. Another thing I like is zfs which is a rock solid platform. Zfs will recognize signs of hard drives failing long before nfts will due to how it interacts and checks the health of hard drives. I am very happy moving away from windows and nfts for my main storage needs. I hope this helps you out if you are in the market for nas units, or looking to build your own. If you have anymore questions feel free to use my contact me page .