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Chapter 14: Mass-Storage Structure

Disk Structure

Disk SchedulingDisk Management

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Background

Magnetic disks and tapes are examples of secondarystorage.Access time for tapes is much slower than for disks.Tapes are currently used for:

BackupStorage of infrequently used informationTransferring info from one system to anotherStoring very large quantities of data

Disks provide the bulk of secondary storage for moderncomputer systems.

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Disk StructureDisk drives are addressed as large 1-dimensionalarrays of logical blocks , where the logical block is thesmallest unit of transfer.

The 1-dimensional array of logical blocks is mappedinto the sectors of the disk sequentially.

Sector 0 is the first sector of the first track on theoutermost cylinder.Mapping proceeds in order through that track, thenthe rest of the tracks in that cylinder, and thenthrough the rest of the cylinders from outermost toinnermost.

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Disk Scheduling The operating system is responsible for using hardwareefficiently 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 arm to move theheads to the cylinder containing the desired sector.Rotational latency is the additional time waiting forthe disk to rotate the desired sector to the disk head.

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Disk Scheduling (cont.)

Minimize seek time

Seek time

seek distanceDisk bandwidth is the total number of bytes transferred,divided by the total time between the first request forservice and the completion of the last transfer.We can improve access time and bandwidth byscheduling the servicing of disk I/O requests in a goodorder.

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Disk Scheduling (Cont.)

Several algorithms exist to schedule the servicing of diskI/O requests.We illustrate them with a disk queue with requests for I/Oto blocks on cylinders (0-199).

98, 183, 37, 122, 14, 124, 65, 67

in that order. And disk head is initially at cylinder 53.

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1) FCFS

First-come, first-served is the simplest algorithm.

But it does not provide the fastest service.In the example, Illustration shows total head movementof 640 cylinders

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FCFS (cont.)Illustration shows total head movement of 640 cylinders.

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2) SSTF

Shortest-seek-time-first.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 236cylinders.Improvement in performance.Not optimal.

Better: 53, 37 , 14, 65, 67, 98, 122, 124, 183 = 208cylinders.

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SSTF (Cont.)

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3) SCAN

The disk arm starts at one end of the disk, and movestoward the other end, servicing requests until it gets tothe other end of the disk, where the head movement isreversed and servicing continues.Need to know the direction of head movement, inaddition to the heads current position (53).Sometimes called the elevator algorithm .In example, the disk arm is moving toward 0.

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SCAN (Cont.)

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4) C-SCAN

Circular SCAN

Provides a more uniform wait time than SCAN.The head moves from one end of the disk to theother, 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 wrapsaround from the last cylinder to the first one.

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C-SCAN (Cont.)

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5) LOOK and C-LOOK

Versions of SCAN and C-SCANArm only goes as far as the last request in eachdirection, then reverses direction immediately, withoutfirst going all the way to the end of the disk.They LOOK for a request before continuing to move in agiven direction.

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C-LOOK (Cont.)

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exampleWork Queue : 23, 89, 132, 42, 187

there are 200 cylinders numbered from 0 - 199

the disk head stars at number 100 (goingtowards 0)

Starting from the current head position, what isthe total distance (in cylinders) that the disk armmoves to satisfy all the pending requests for

each of the following disk-schedulingalgorithms:FCFS, SSTF, SCAN, LOOK, C-SCAN, C-LOOK

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Selecting a Disk-Scheduling Algorithm

SSTF is common and has a natural appealSCAN and C-SCAN perform better for systems thatplace a heavy load on the disk, because they are lesslikely to have a starvation problem.Performance depends on the number and types ofrequests.Requests for disk service can be influenced by the file-allocation method (if they are contiguously allocated ornot).

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Selecting a Disk-Scheduling Algorithm

(cont.)The location of directories is also important.Since every file must be opened to be used,

and opening it requires searching the directorystructure, the directories will be accessedfrequently.

The disk-scheduling algorithm should bewritten as a separate module of the operatingsystem, allowing it to be replaced with adifferent algorithm if necessary.Either SSTF or LOOK is a reasonable choicefor the default algorithm.

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ExerciseSuppose that a disk drive has 5000 cylindersnumbered 0 to 4999. the drive is currentlyserving a request at cylinder 143, and theprevious request was at cylinder 125. Thequeue of pending requests, in FIFO order is:

86,1470, 913, 1774, 948,1509, 1022, 1750, 130Starting from the current head position, what isthe total distance (in cylinders) that the disk armmoves to satisfy all the pending requests foreach of the following disk-schedulingalgorithms:

FCFS, SSTF, SCAN, LOOK, C-SCAN, C-LOOK

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Disk scheduling (cont.)

Whenever a process needs I/O to or from disk, it issuesa system call to the OS. The request specifies thefollowing information:

Whether this operation is input or output.What the disk address for the transfer is.What the memory address for the transfer is.What the number of bytes to be transferred is.

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Disk Management1- Disk Formatting:

Low-level formatting , or physical formatting Dividing a disk into sectors that the disk controllercan read and write.Low-level formatting fills the disk with a special datastructure for each sector consists of:

HeaderData area (usually 512 bytes in size)Trailer

Header and trailer contain information used by thedisk controller such as sector number and error-correcting code (ECC).

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Disk Formatting (cont.)Most hard disks are low-level formatted at the factory asa part of the manufacturing process.

Formatting a disk with a larger sector size means thatfewer sectors can fit on each track, So, fewer headersand trailers are written on each track increase thespace available for user data.

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Disk Formatting (cont.)

To use a disk to hold files, the operating system stillneeds to record its own data structures on the disk. Itdoes that in 2 steps:1. Partition the disk into one or more groups of

cylinders.2. Logical formatting or creation of a file system. In this

step , the OS stores the initial file-system datastructures onto the disk (maps of free and allocatedspace FAT and an initial empty directory)

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Block Addressing

8 blocks / track

01

2

34

5

67 0

12

3 4

56

70

3

1

6

4

7

25

No interleaving Single interleaving Double interleaving

Interleaving: is used to give controller time to transfer data tomemory.

Block numbering: When the disk is formatted, the blocks are

numbered to take account of the interleave factor.

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Disk Management

2- Boot Block For a computer to start running, it needs to have aninitial program to run.The bootstrap program is usually stored in ROMbecause ROM :

Needs no initialization

Is at fixed locationIs Read only, cant be infected by virus.

Problem: changing bootstrap code requires changing the

ROM hardware chips.Most systems store a tiny bootstrap loader program inthe boot ROM, whose only job is to bring in a fullbootstrap program from disk.

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Boot Block (cont.)

Full bootstrap program is stored in a partition calledthe boot blocks at a fixed location on the disk.Boot block initializes system.Boot disk or system disk : A disk that has a bootpartition.Full bootstrap code may be small. (example: MS-DOS uses one 512-byte for its boot program).

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MS-DOS Disk Layout

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Disk Management

3- Bad BlocksBecause disk have moving parts and smalltolerances (disk head flies just above the disk

surface), they are prone to failure.Complete failure: replace diskIncomplete failure: one or more sectors

become defective (bad blocks)Bad blocks are handled in a variety of ways:

On simple disks:manually (format, chkdsk)

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Bad Blocks (cont.)More sophisticated disks:

1. Methods such as sector sparing (forwarding) used to handle bad blocks: controller replace eachbad sector logically with one of the spare sectors.

Example:OS tries to read logical block 87.Controller calculates the ECC and finds that the

sector is bad.When system is rebooted, a special command isrun to tell the controller to replace the bad sectorwith a spare.After that, whenever the system requests logicalblock 87, the sector is translated into thereplacement sectors address by the controller.

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Bad Blocks (cont.)2. Replace a bad sector by sector slipping .

Example:Suppose that logical block 17 becomes defective, and the

first available spare follows sector 202.Sector slipping would remap all the sectors from 17 to202, moving them all down one spot.

(sector 202 copied to spare, sector 201 to 202.. Sector 18to 19.)Slipping sectors in that way frees up the space of sector

18, so sector 17 can be mapped to it.Data in bad block are usually lost, thus, whatever file wasusing that block must be repaired (restored from backup

tape).