zfs-style scrub only reads allocated blocks and skips unallocated blocks; if the pool is mostly empty it can complete very quickly. Layered storage systems wit

zfs-style scrub only reads allocated blocks and skips unallocated blocks; if the pool is mostly empty it can complete very quickly. Layered storage systems with a RAID layer that makes N disks look like one big disk generally don't have visibility into which blocks are free and which are allocated so they must "scrub" all the disks on initialization and repair even if only 1% is used.