Ch 13. Mass-Storage Structure

1. Ch 13. Mass-Storage Structure 2014-10-08

2. 13.1 Disk Structure • Disk Structure • characteristic of H/W track sector cylinder OS Ch.13 Mass Storage Structure

3. 13.1 Disk Structure 20 ~ 1500 track / disk 4 ~ 32 sector / track 32 ~ 4k bytes / sector number of track = number of cylinder total number of sector = surface * cylinder * number of sector access time of Disk = seek time + latency time + transmission time • seek time • searching time of correct cylinder • latency time • time that read-write head are moved to correct sector • transmission time • time that information are transmitted OS Ch.13 Mass Storage Structure

4. 13.2 Disk Scheduling • FCFS scheduling • Most simplest form of scheduling • ex) Track # of Disk Queue Queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 0 14 37 53 65 67 98 122 124 183 199 Figure 13.1 FCFS disk scheduling Head Movement of 640 Track OS Ch.13 Mass Storage Structure

5. 13.2 Disk Scheduling • Shortest Seek Time First ( SSTF ) • most nearest demand first ( generally used ) • about 1/3 compared with FCFS • 236 track head movement • Possibility of starvation Queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 0 14 37 53 65 67 98 122 124 183 199 Figure 13.2 SSTF disk scheduling OS Ch.13 Mass Storage Structure

6. Figure 13.3 SCAN disk scheduling 13.2 Disk Scheduling Scan ( Elevator algorithm ) • Head goes and comes again from inside of disk to outside • Good for large amount of disk usage ( usually long waiting ) Queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 0 14 37 53 65 67 98 122 124 183 199 OS Ch.13 Mass Storage Structure

7. Figure 13.4 C-SCAN disk scheduling 13.2 Disk Scheduling • C-SCAN ( Circular SCAN ) • If reached at one side of disk, scan again at start point. • Uniform wait time Queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 0 14 37 53 65 67 98 122 124 183 199 OS Ch.13 Mass Storage Structure

8. 13.2 Disk Scheduling • Look : Look for request • Observe the direction of scan • If request come, continue scanning or change the direction. • C - Look  Directory or Index block : Center of the disk. ( ∵ to reduce the head movement ) Queue = 98, 183, 37, 122, 14, 124, 65, 67 head starts at 53 0 14 37 53 65 67 98 122 124 183 199 OS Ch.13 Mass Storage Structure Figure 13.5 C-LOOK disk scheduling

9. 13.3 Disk Management • Disk Management • Disk Formatting • before the computer make use of disk, • the disk broken into the sector ( Physical Formatting ) • Boot Block • O.S Kernel exist -> Bootstrap program -> computer power on -> initial program load -> O.S operating • Bad Block • Disk have moving parts and small tolerance (head floats …. ) • Disk of bad block from the factory ( unreadable / unwritable ) • searching method of manual and format command OS Ch.13 Mass Storage Structure

10. 13.4 Swap-space Management • Swap-space use • Used to hold the entire process image • Paging systems may simply store pages • Load can be spread over the system’s I/O device • Swap-space location • Normal file system • Separate disk partition • Swap-space management • 4.3 BSD : enough space is set • Solaris : thrown away if selected for pageout OS Ch.13 Mass Storage Structure