Why would I consider a 5 disk raidz2 (5x3TB WD Reds)? Only one reason really, my mobo only has 6 sata ports. So I can build a 5 disk raidz2, and have one port left over to copy the data back to the pool from my backup. And I can use the 6th sata port for zfs replicating snapshots of the pool to a pair of very large drive I will rotate offsite. I keep reading that with lz4 enabled, the "non-optimal" sizing of a odd numbered raidz2 goes away. But I haven't successfully googled why this is....
Debating a 5 disk raidz2
Debating a 5 disk raidz2