In this chapter, we begin a discussion of file systems at the lowest level: the structure of secondary storage. We first describe the physical structure of hard disks and magnetic tapes. We then describe disk-scheduling algorithms, which schedule the order of disk I/Os to maximize performance. Next, we discuss disk formatting and management of boot blocks, damaged blocks, and swap space. We conclude with an examination of the structure of RAID systems. | Chapter 14: Mass-Storage Systems I Disk Structure I Disk Scheduling I Disk Management I Swap-Space Management I RAID Structure I Disk Attachment I Stable-Storage Implementation I Tertiary Storage Devices I Operating System Issues I Performance Issues Operating System Concepts Silberschatz, Galvin and Gagne 2002 Disk Structure I Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. I The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. ✦ Sector 0 is the first sector of the first track on the outermost cylinder. ✦ Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. Operating System Concepts Silberschatz, Galvin and Gagne 2002 Disk Scheduling I The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth. I Access time has two major components ✦ Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. ✦ Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. I Minimize seek time I Seek time ≈ seek distance I Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer. Operating System Concepts Silberschatz, Galvin and Gagne 2002 Disk Scheduling (Cont.) I Several algorithms exist to schedule the servicing of disk I/O requests. I We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53 Operating System Concepts Silberschatz, Galvin and Gagne 2002 FCFS Illustration shows total head movement of 640 cylinders. Operating System Concepts Silberschatz, Galvin and .
