When a disk fails, Data ONTAP can continue to serve data, but it must reconstruct the data from the failed disk using RAID parity. When this happens, the affected RAID group is said to be in degraded mode. The performance of a storage system with one or more RAID groups in degraded mode is decreased.
After the failed disk is reconstructed to a spare, the RAID group returns to normal mode.
If two disks have failed in a RAID-DP group, the RAID group goes into double-degraded mode.
After the offline disk is brought back online, the RAID group returns to normal mode.