Disk Organization

8/2/2019 Disk Organization

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Topics Covered

Physical Disk Organization

Example

Disk Scheduling Algorithms

Research Work

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Physical disk organization

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To read or write, the disk head must bepositioned on the desired track and at thebeginning of the desired sector

Seek time is the time it takes to position thehead on the desired track

Rotational delay or rotational latency isthe additional time its takes for the

beginning of the sector to reach the headonce the head is in position

Transfer time is the time for the sector to

pass under the head 4




Access time= seek time + rotational latency + transfer time

Efficiency of a sequence of disk accesses

strongly depends on the order of the requests Adjacent requests on the same track avoid

additional seek and rotational latency times

Loading a file as a unit is efficient when thefile has been stored on consecutive sectors onthe same cylinder of the disk

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Timing of a Disk I/O Transfer

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Example:Two single-sector disk requests

Assume-- average seek time = 10 ms

-- average rotational latency = 3 ms

-- transfer time for 1 sector = 0.01875 ms Adjacent sectors on same track

-- access time = 10 + 3 + 2*(0.01875) ms =

13.0375 ms

Random sectors

-- access time = 2*(10 + 3 + 0.01875) ms =

26.0375 ms

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Several algorithms exist to schedule the servicing of

I/O requests. These are

Random FIFO or FCFSPRI SSTF

SCAN C-SCAN

N-Step-SCAN FSCAN

LOOK C-LOOK

Random – Just a benchmark for comparison

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PRIPriority given to requests

Scheduling largely outside of disk management controlShort batch jobs may have higher priorityThis provides good interactive response time


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Evaluating the policies

To evaluate the policies, we will use a random

sequence of track accesses Head starting at100

55 58 39 18 90 160 150 38 184

Calculate how many track traversal each policy

will require to finish the sequence


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First Come First Served Scheduling or FIFO

Simplest form

Not provide Fastest Service Direction of movement from left to right

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FIFO : process requests in the order they arrive

Queue =55 58 39 18 90 160 150 38 184

Head start at 100

Total track traversed 498


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Disk Scheduling Algorithms The Shortest Service Time First (SSTF)

Service all the requests close to the current head

position From requests currently in the queue, choose the

request that minimizes movement of the arm

(read/write head)

May cause starvation

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The Shortest Service Time First (SSTF)

Queue =55 58 39 18 90 160 150 38 184

90 58 55 39 38 18 150 160 184



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Disk Scheduling Algorithms SCAN

Arm moves in one direction only until it reachesthe last request in that direction

Then the arm reverses and repeats

Avoids starvation

Also called elevator algorithm

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SCAN Queue =55 58 39 18 90 160 150 38 184

150 160 184 90 58 55 39 38 18



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C-SCAN (Circular SCAN)

Like SCAN, but in one direction only

Then returns arm to the opposite side andrepeats

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C-SCAN (Circular SCAN)

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Queue =55 58 39 18 90 160 150 38 184

150 160 184 18 38 39 55 58 90



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Divide queue into N-request segments

Use SCAN on each

New requests are added to the rear of the queueto form a new N-request segment

Reduces maximum waiting time in a high-

volume situation Causes head to move more frequently from one

cylinder to the next

N-step-SCAN


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Disk Scheduling Algorithms FSCAN Like N-step-SCAN but with two queues

One queue fills while the other is processed

using SCANLOOK

Version of Scan

C-LOOK Version of C-SCAN

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Disk Organization

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