IBM N series Hardware Manual page 184

During disk failure, the storage system replaces the failed disk with a spare and reconstructs
data. If a disk fails, the storage system runs the following actions:
1. The storage system replaces the failed disk with a hot spare disk. If RAID-DP is enabled
and double-disk failure occurs in the RAID group, the storage system replaces each failed
disk with a separate spare disk. Data ONTAP first attempts to use a hot spare disk of the
same size as the failed disk. If no disk of the same size is available, Data ONTAP replaces
the failed disk with a spare disk of the next available size up.
2. The storage system reconstructs, in the background, the missing data onto the hot spare
disks.
3. The storage system logs the activity in the /etc/messages file on the root volume.
With RAID-DP, these processes can be carried out even if two disks simultaneously fail in a
RAID group.
During reconstruction, file service can slow down. After the storage system is finished
reconstructing data, replace the failed disks with new hot spare disks as soon as possible.
Hot spare disks must always be available in the system.
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IBM System Storage N series Hardware Guide