ZFS is designed for data integrity from top to bottom. RAID-Z, the software RAID that is part of ZFS, offers single parity protection like RAID 5, but without the “write hole” vulnerability thanks to the copy-on-write architecture of ZFS. The additional levels RAID-Z2 and RAID-Z3 offer double and triple parity protection, respectively. A software mirror option is also available. The FreeNAS Volumes screen lists each possible parity arrangement based on the number of disks you select when creating a new volume.
Every ZFS filesystem is also verified with checksums from top to bottom to ensure data integrity. If inconsistencies are found, parity blocks can be used to repair corrupt data. A regular scrub is turned on by default and can be rescheduled or configured from the web interface.