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Module 13: Secondary-Storage.• Disk Structure.• Disk Scheduling.• Disk Management.• Swap-Space Management.• Disk Reliability.• Stable-Storage Implementation.• Tertiary Storage Devices.• Operating System Issues.• Performance Issues. 13.1 Silberschatz and Galvin 1999 Disk Structure.• Disk drives are addressed as large 1-dimensional arrays of. logical blocks, where the logical block is the smallest unit of. transfer• 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 13.2 Silberschatz and Galvin 1999 Disk Scheduling.• The operating system is responsible for using hardware. efficiently — for the disk drives, this means having a fast. access time and disk bandwidth• 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• Minimize seek time.• Seek time seek distance.• 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 13.3 Silberschatz and Galvin 1999 Disk Scheduling (Cont.).• Several algorithms exist to schedule the servicing of disk I/O. requests• We illustrate them with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67. Head pointer 53. 13.4 Silberschatz and Galvin 1999 FCFSIllustration shows total head movement of 640 cylinders 13.5 Silberschatz and Galvin 1999 SSTF.• Selects the request with the minimum seek time from the. current head position• SSTF scheduling is a form of SJF scheduling; may cause. starvation of some requests• Illustration shows total head movement of 236 cylinders 13.6 Silberschatz and Galvin 1999 SSTF (Cont.). 13.7 Silberschatz and Galvin 1999 SCAN.• The disk arm starts at one end of the disk, and moves toward. the other end, servicing requests until it gets to the other end. of the disk, where the head movement is reversed and. servicing continues• Sometimes called the elevator algorithm• Illustration shows total head movement of 208 cylinders 13.8 Silberschatz and Galvin 1999 SCAN (Cont.). 13.9 Silberschatz and Galvin 1999 C-SCAN.• Provides a more uniform wait time than SCAN• The head moves from one end of the disk to the other servicing requests as it goes. When it reaches the other end,. however, it immediately returns to the beginning of the disk,. without servicing any requests on the return trip• Treats the cylinders as a circular list that wraps around from. the last cylinder to the first one 13.10 Silberschatz and Galvin 1999 C-SCAN (Cont.). 13.11 Silberschatz and Galvin 1999 C-LOOK.• Version of C-SCAN.• Arm only goes as far as the last re
