Overview
A rebuild reconstructs the missing data or parity onto a replacement drive. The amount of work depends on the RAID layout, array occupancy, drive size, controller policy and workload during the rebuild.
Engineering considerations
A large drive does not imply a fixed rebuild duration. Sequential rebuild throughput can be much higher than the effective rate available when the array is serving production I/O. Parity RAID may also require reconstruction calculations rather than simple copying.
How to plan it
During a degraded period, the array may have reduced performance and reduced fault tolerance. A second failure can have consequences that depend on the RAID level and exact failure location. RAID 6 provides an additional parity position compared with RAID 5, while RAID 10 uses mirrored members.
Design implications
Use the RAID Rebuild Time Calculator as a first-order estimate. Measure actual controller rebuild rates and understand the vendor's rebuild priority and background-task controls before using the number as an operational SLA.
Operational considerations
RAID is not a backup. A rebuild cannot recover data that was already deleted, corrupted or overwritten.