IBM N Series Hardware Manual page 166

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 in the event of simultaneous failure
of two disks 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