Ch 13. Mass-Storage Structure

04/20/23

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13.1 Disk Structure Disk Structure

characteristic of H/W

sector

track

cylinder

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13.1 Disk Structure

20 ~ 1500 track / disk4 ~ 32 sector / track32 ~ 4k bytes / sectornumber of track = number of cylindertotal number of sector = surface * cylinder * number of

sectoraccess 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

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1. FCFS scheduling Most simplest form of scheduling ex) Track # of Disk Queue

13.2 Disk Scheduling

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

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13.2 Disk Scheduling

2. 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

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13.2 Disk Scheduling

Scan ( Elevator algorithm )Head goes and comes again from inside of disk to outsideGood 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

Figure 13.3 SCAN disk scheduling

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13.2 Disk Scheduling

4. 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

Figure 13.4 C-SCAN disk scheduling

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13.2 Disk Scheduling5. Look : Look for request

Observe the direction of scan

If request come, continue scanning or change the direction.

6. 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

Figure 13.5 C-LOOK disk scheduling

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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

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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