RAID4 provides single-parity disk protection against single-disk failure within a RAID group. If an aggregate is configured for RAID4 protection, Data ONTAP reconstructs the data from a single failed disk within a RAID group and transfers that reconstructed data to a spare disk.
The minimum number of disks in a RAID4 group is two: at least one data disk and one parity disk. However, for non-root aggregates with only one RAID group, you must have at least 3 disks (two data disks and one parity disk).
If there is a single data or parity disk failure in a RAID4 group, Data ONTAP replaces the failed disk in the RAID group with a spare disk and uses the parity data to reconstruct the failed disk’s data on the replacement disk. If no spare disks are available, Data ONTAP goes into degraded mode and alerts you of this condition.