RAIDZ

• RAIDZ-1 A variation on RAID-5, single parity. Requires at least 3 disks.
• RAIDZ-2 A variation on RAID-5, double parity. Requires at least 4 disks.
• RAIDZ-3 A variation on RAID-5, triple parity. Requires at least 5 disks.

Space efficiency

Not possible now. It is possible to extend space by adding similar vdev (raidz based)

Expanding usable capacity on raidz pools by adding devices #12467 RAIDZ Expansion feature #12225

Check features:

zpool get all backuppool