What would be the drawback of having 7 of my drives (2TB) in a RAIDZ2. FreeNAS does not like the 7 drives, it wants 6. But when it says optimal does that mean it will perform slower with 7 drives?
Hi Arvo,
Short answer: Yes, a 7-drive RAIDZ2 will probably be slower than the 6-drive RAIDZ2.
Computers really like the number two, powers of two, and things that are divisible nicely by twos and powers of two. So it likes 6 disks because it has 4 drives worth of data (2^2) and 2 of parity.
Let's say you write a 128KB stripe of data to that array. We'll ignore the parity drives for now; with four drives holding data, it divides very nicely into four chunks of 32KB, which go to each disk. And if you're using 4KB-sector drives, each chunk takes up eight sectors. Nice round numbers.
But if you have a 7-drive RAIDZ2, you're going to have five data disks. 128KB/5 = 25.6KB per drive. Uh oh, that didn't divide nicely. You're now writing 6 full sectors (24KB) and one partial sector (1.6KB) to each disk.
With all that said
If you're only connecting over a single gigabit link, you'll probably be able to saturate the link with either configuration. But the 6-drive RAIDZ2 is the "recommended setup"