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When a disk fails unexpectedly, a RAID array copies data to a spare drive while the failed one is replaced. Data is then reassembled on the new drive using RAID algorithms and parity data. RAID rebuilds can also be proactive if the RAID array detects that a hard drive is about to fail. During the rebuild, the performance of some applications or processes may be negatively affected by latency.
RAID rebuild times are an important concern for storage administrators. Higher capacity drives or multiple failed drives can result in longer rebuild times that take days.