ZFS RAID Size and Reliability Calculator
If you need to calculate the usable storage size for any given ZFS RAIDZ configuration, I recommend using this calculator. It was found on the TrueNAS Community Forum.
RAID
Why RAID 5 stops working in 2009
Why RAID 5 stops working in 2009 in a short summary:
TLDR: Hard disk manufactures use something called unrecoverable read error rate, or URE for