Q. Remapping of bad blocks by sector sparing or else sector slipping could influence performance. Presume that the drive in Subsequent Exercise has a total of 100 bad sectors at random locations and that every bad sector is mapped to a spare that is located on a different track but within the same cylinder. Estimation the number of I/Os per second and the effective transfer rate for a random-access workload consisting of 8-kilobyte reads with a queue length of 1 (that is the choice of scheduling algorithm isn't a factor). What is the consequence of a bad sector on performance?
Answer: Since the disk holds 22,400,000 sectors the probability of requesting one of the 100 remapped sectors is very small. An instance of a worst-case event is that we attempt to read say an 8 KB page however one sector from the middle is defective and has been remapped to the worst possible location on another track in that cylinder. In this situation the time for the retrieval could be 8 ms to seek plus two track switches and two full rotational latencies. It is probable that a modern controller would read all the requested good sectors from the original track before switching to the spare track to retrieve the remapped sector and thus would incur only one track switch and rotational latency. Therefore the time would be 8ms seek + 4.17 ms average rotational latency + 0.05 ms track switch + 8.3 ms rotational latency + 0.83 ms read time (8 KB is 16 sectors 1/10 of a track rotation). Therefore the time to service this request would be 21.8ms giving an I/O rate of 45.9 requests per second as well as an effective bandwidth of 367 KB/s. For a strictly time-constrained application this might matter but the overall impact in the weighted average of 100 remapped sectors and 22.4 million good sectors is nil.