Raidz2 (survives 2 lost drives) is slower and more complicated than raidz (survives 1 drive loss). The advantage of keeping a powered-off spare (that's good) is

Raidz2 (survives 2 lost drives) is slower and more complicated than raidz (survives 1 drive loss). The advantage of keeping a powered-off spare (that's good) is there zero wear and (near) zero power usage. The odds of 2 drive failures in rapid succession without temperature rise or SMART issues is around similar odds of a jet turbine breaking down. Raidz3 (survives 3 lost drives) is pure insanity in terms of computational demands. Better off splitting up pools, using multiple systems, or using multiple copy systems up the stack like with Ceph on multiple, simple RAID1 or RAID10 storage nodes.