problem 1

Disk requests come in to disk driver for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. A seek takes 6 mes per cylinder moved. How much seek time is required for

(a) First-come, first served.

(b) Closest cylinder next.

(c) Elevator algorithm (initially moving upward).

In all cases, the arm is initially at cylinder 20.

problem 2

A slight change of the elevator algorithm for scheduling disk requests is to always scan in the similar direction, In what respect is this modified algorithm better than elevator algorithm?

problem 3

A RAID could fail if two or more of its drives crash within the short time interval. Assume that probability of one drive crashing in a given hour is p. What is the probability of a k-drive RAID failing in a given hour?

problem 4

Consider magnetic disk consisting of 16 heads and 400 cylinders. This disk is divided into four 100-cylinder zones with the cylinders in different zones containing 160, 200, 240, and 280 sectors, respectively. Suppose that each sector contains 512 bytes, average seek time between adjacent cylinders is 1 msec, and disk rotates at 7200 RPM. Compute the (a) disk capacity, (b) optimal track skew, and (c) maximum data transfer rate.

problem 5

Some operating systems give a system call rename to give a file a new name. Is there any difference at all between using this call to rename a file and just copying the file to a new file with the new name, followed by deleting the old one?

problem 6

One way to use contiguous allocation of the disk and not suffer from holes is to compact disk every time a file is removed. As all files are contiguous, copying a file needs a seek and rotational delay to read file, followed by transfer at full speed. Writing file back needs the same work. Suppose a seek time of 5 msec, a rotational delay of 4 msec, a transfer rate of 8 MB/sec, and average file size of 8 KB, (a) how long does it take to read a file into main memory then prepare it back to disk at new location? (b) Using these numbers, how long will it take to compact half of a 16-GB disk? (c) Dose compacting the disk ever make any sense?

problem 7

Free disk space could be kept track of using a free list or a bit map. Disk addresses need D bits. For a disk with B blocks, F of which are free, state condition under which free list uses less space than bitmap. For D having value 16 bits, express your answer as a percentage of the disk space that should be free.

problem 8

What will happen if the bitmap or free list containing information about free disk blocks was totally lost due to a crash? Is there any way to recover from this disaster, or is it bye-bye disk? Discuss your answers for a UNIX and the FAT-16 file system separately.