Work out raid rebuild time instantly with clear inputs, formula shown and shareable results.
Parity RAID rebuild must read every surviving disk in full to reconstruct the failed one, so read volume scales with array width while write volume is one disk. At 120 MB/s a 16 TB disk takes about 37 hours to write, and the whole array is degraded throughout — which is why RAID 5 on large modern disks is widely considered unsafe.
Rebuild time
parity RAID reads (disks - parity) x capacity and writes one capacity; time = max(read, write) bytes / rebuild rate
Rebuild takes over a day, during which a second failure or an unrecoverable read error destroys the array. With multi-terabyte disks the probability of hitting one is no longer negligible.
Only by accepting more impact on production I/O, since rebuild throughput is throttled to preserve it. Faster rebuild is one of the main advantages of RAID 10 and of declustered parity.