Q. More than one disk drive is able to be attached to a SCSI bus. Especially a fast wide SCSI-II bus can be connected to at most 15 disk drives. Remember that this bus has a bandwidth of 20 megabytes per second. At any time merely one packet can be transferred on the bus between some disk's internal cache and the host. Nevertheless a disk can be moving its disk arm while some other disk is transferring a packet on the bus. As well a disk can be transferring data between its magnetic platters and its internal cache while some other disk is transferring a packet on the bus. Considering the transfer rates that you computed for the various workloads in subsequent discuss how many disks can be used effectively by one fast wide SCSI-II bus.
Answer:
For 8 KB random Input/output on a lightly loaded disk where the random access time is computed to be about 13 ms (see subsequent exercise) the effective transfer rate is about 615 MB/s. In this condition 15 disks would have an aggregate transfer rate of less than 10 MB/s which shouldn't saturate the bus. For 64 KB random examine to a lightly loaded disk the transfer rate is about 3.4 MB/s therefore five or fewer disk drives would saturate the bus. For 8 KB reads with a large sufficient queue to reduce the average seek to 3 ms the transfer rate is about 1 MB/s thus the bus bandwidth may be adequate to accommodate 15 disks.