CA 714 CA - Homework #2

• The example on page 744 assumed the RPM, seek time, MTTF, bandwidth, and cost per GB was similar for the large disk drive and the small disk drive.   Go to a web site and find the best cost-performance 3.5 inch drive and the best cost-performance 2.5 inch dirve.  Assume in a single enclosure you can pack 8 3.5 inch "half height" dirves (1.7 inches high), 12 3.5 inch "low profile" (1.0 inches high), or 36 2.5 inch dirves.  Assume that all have a SCSI interface so that you can connect up to 15 drives on a string.  Design the RAID organization and calculate the cost-performance and reliability as in the example on page 752.  Use parameters from that example if you cannot find more recent information from web sites.

• Assume that we have the following two magnetic-disk configurations: a single disk and an array of four disks.  Each disk has 20 surfaces, 6526 tracks per surface and 171 sectors/track.  Each sector holds 512 bytes, and it revolves at 10,033 RPM.  Use the seek time formula in the fallcy starting on page 768, including the equations in Figure 7.51 (page 768).  The time to switch between surfaces is the same as to move the arm one track.  In the disk array all the spindles are synchronized - sector 0 in every disk rotates under the head at the exact same time - and the arms on all four disks are always over the same track.  The data is "striped" across all four disks, so four consecutive sectors on a single-disk system will be spread one sector per disk in the array.  The delay of the disk controller is 1ms per transaction, either for a single disk or for the array.  Assume the performance of the I/O system is limited only by the disks and that there is a path to each disk in the array.  Calculate the performance in both I/Os per second and megabytes per second of these two disk organizations, assuming the request pattern is random reads of 4 KB of sequential sectors.  Assme the 4 KB are aligned under the same arm on each disk in the array.