Post on 06-Apr-2018
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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Physical disk organization
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Physical disk organization
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
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Physical disk organization
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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Disk Scheduling Algorithms
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
Disk Scheduling Algorithms
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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
Disk Scheduling Algorithms
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Disk Scheduling Algorithms
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
Disk Scheduling Algorithms
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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
Total track traversed 248
Disk Scheduling Algorithms
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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
Total track traversed 250
Disk Scheduling Algorithms
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Disk Scheduling Algorithms
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
Total track traversed 322
Disk Scheduling Algorithms
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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
Disk Scheduling Algorithms
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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 Scheduling Algorithms
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