department of computer and it engineering university of kurdistan operating systems disk management...
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Department of Computer and IT EngineeringDepartment of Computer and IT EngineeringUniversity of KurdistanUniversity of Kurdistan
Operating systemsDisk Management
By: Dr. Alireza AbdollahpouriBy: Dr. Alireza Abdollahpouri
Hard Disk Drives
Western Digital Drivehttp://www.storagereview.com/guide/
Read/Write HeadSide View
Mechanics of Disks
Platterscircular platters covered with magnetic material to provide
nonvolatile storage of bits
Tracksconcentric circles on a
single platter
Sectorssegment of the track circle –
usually each contains 512 bytes –separated by non-magnetic gaps.
The gaps are often used to identifybeginning of a sector
Cylinderscorresponding tracks on the different
platters are said to form a cylinder
Spindleof which the platters
rotate around
Disk headsread or alter the
magnetism (bits) passing under it. The heads are
attached to an arm enabling it to move across the platter
surface
Hard Disk Drives
+ Rotational delay
+ Transfer time
Seek time
Disk access time =
+ Other delays
Disk platter
Disk arm
Disk head
Disk Access Time
Measure Disk Performance• Transfer rate is rate at which data flows
between drive and computer• Positioning time (random-access time) is time
– The time to move disk arm to desired cylinder (seek time)
– and the time for desired sector to rotate under the disk head (rotational latency)
Magnetic Disks Platters range from .85” to 14” (historically)
Commonly 3.5”, 2.5”, and 1.8” Range from 30GB to 3TB per drive Performance
Transfer Rate – theoretical – 6 Gb/sec Effective Transfer Rate – real –
1Gb/sec Seek time from 3ms to 12ms – 9ms
common for desktop drives Average seek time measured or
calculated based on 1/3 of tracks Latency based on spindle speed
1/(RPM * 60) Average latency = ½ latency
(From Wikipedia)
Magnetic Disk Performance Access Latency = Average access time = average seek time +
average latency For fastest disk 3ms + 2ms = 5ms For slow disk 9ms + 5.56ms = 14.56ms
Average I/O time = average access time + (amount to transfer / transfer rate) + controller overhead
For example to transfer a 4KB block on a 7200 RPM disk with a 5ms average seek time, 1Gb/sec transfer rate with a .1ms controller overhead = 5ms + 4.17ms + 4KB / 1Gb/sec + 0.1ms = 9.27ms + 4 / 131072 sec = 9.27ms + .12ms = 9.39ms
Disk Structure
Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer
The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially Sector 0 is the first sector of the first track on the outermost
cylinder Mapping proceeds in order through that track, then the rest of
the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost
Logical to physical address should be easy Except for bad sectors Non-constant # of sectors per track via constant angular
velocity
Disk Scheduling Disk can do only one request at a time; What
order do you choose to do queued requests?
Seek time seek distance Several algorithms exist to schedule the servicing
of disk I/O requests
3210752 HeadUserRequests
cylinder # of requested block
Disk Scheduling (Cont.)
There are many sources of disk I/O request OS System processes Users processes
I/O request includes input or output mode, disk address, memory address, number of sectors to transfer
OS maintains queue of requests, per disk or device Idle disk can immediately work on I/O request, busy disk means work must queue
Optimization algorithms only make sense when a queue exists Note that drive controllers have small buffers and can manage a queue of I/O requests
(of varying “depth”) Several algorithms exist to schedule the servicing of disk I/O requests The analysis is true for one or many platters We illustrate scheduling algorithms with a request queue (0-199)
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
Disk Scheduling: FCFS Fair among requesters, but order of arrival
may be to random spots on the disk Very long seeks
Example
در اين روش سيلندرهاي متقاضي
به ترتيب درخواست شان سرويس دهي
مي شوند. بعبارتي هر درخواست در صف اجرا قرار مي گيرد.
ساده ترين روش است اما كارآيي چنداني
.ندارد
در اين روش سيلندرهاي متقاضي
به ترتيب درخواست شان سرويس دهي
مي شوند. بعبارتي هر درخواست در صف اجرا قرار مي گيرد.
ساده ترين روش است اما كارآيي چنداني
.ندارد
Disk Scheduling: SSTF در اين روش، هر لحظه سيلندر متقاضي كه به
محل هد در آن لحظه نزديكتر باشد مورد پردازش قرار مي گيرد.
Disk Scheduling: SCAN• The disk arm starts at one end of the disk, and moves toward the other end,
servicing requests until it gets to the other end of the disk, where the head
movement is reversed and servicing continues. Sometimes called the
elevator algorithm
Disk Scheduling: C-SCAN The head moves from one end of the disk to the other, 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 wraps around from the last cylinder to the first one
C-LOOK Arm only goes as far as the last request in each direction, then
reverses direction immediately, without first going all the way to the end of the disk
Total number of cylinders?
مثالدیسک یک کنید درخواست 200فرض صف و داشته شیار
درخواستهای شیار . Randomدیسک دارد خود در را(track) عبارتند دریافت ترتیب به شده خواست در های
از : 55 ,58 ,39,18,90,160,38,184: زیر حاالت از یک هر در
دیگر شیار به شیار از حرکت زمان طول 4اگر ثانیه میلیشیار روی در ابتدا در دیسک بازوی و قرار 100بکشد،
باشد : داشته متوسط طول و خواستها در به دهی سرویس ترتیب
Seek ؟ است چقدر ؟ بود خواهد چقدر جستجو زمان کل
) روش از کنید .FIFOالف استفادهالگوریتم( ب شود . SSTFاز استفادهج. شود ( استفاده آسانسور الگوریتم ازالگوریتم ( د شود . C-SCANاز استفاده
c-scan و scanفرض کنید در روش جهت اولیه حرکت به سمت افزایش شماره شیار می باشد.
request in queue
100
55
58
39
18
90
160
150
38
184
order of service
0
1
2
3
4
5
6
7
8
9
FIFO
پیموده track تعداد های شده
0
45
3
19
21
72
70
10
112
146
55.3Average Seek Length 498 های پیموده trackتعداد =
FIFO
100
55 58
39
18
90
160150
38
184
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
trac
k ره
شما
= زمان جستجو کل
498 های پیموده trackتعداد = 498 * 4 (ms) =1992 msec
FIFO
order of service
request serviced
track تعدادپیموده های
شده
0 100
1 90 10
2 58 32
3 55 3
4 39 16
5 38 1
6 18 20
7 150 132
8 160 10
9 184 24
= زمان جستجو کل
های trackتعداد =پیموده
248
248 * 4 (ms) = 299 msec
27.5Average Seek Length
SSTF
SSTF
100
90
58 55
39 38
18
150
160
184
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
ارشی
رهشما
SSTF
order of service
request serviced
track تعدادپیموده های
شده
0 100
1 90 10
2 58 32
3 55 3
4 39 16
5 38 1
6 18 20
7 150 132
8 160 10
9 184 24
request in queue
100
55
58
39
18
90
160
150
38
184
order of service
0
1
2
3
4
5
6
7
8
9
request serviced
27.8Average
Seek Length
100
150
160
184
90
58
55
39
38
18
SCAN or ELEVATOR آسانسور))
= زمان جستجو کل
250 های پیموده trackتعداد =
250 * 4 (ms) = 1000 msec
های پیموده track تعدادشده
50102494323
161
20
order of service
request serviced
0 100
1 150
2 160
3 184
4 90
5 58
6 55
7 39
8 38
9 18
SCAN
100
150160
184
90
58 55
39 38
18
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
یار
شره
شما
SCAN or ELEVATOR آسانسور))
order of service
request serviced
track تعدادپیموده های
شده
0 100
1 150 50
2 160 10
3 184 24
4 18 166
5 38 20
6 39 1
7 55 16
8 58 3
9 90 32
35.8Average Seek Length
= زمان جستجو کل
322 های پیموده trackتعداد =
322 * 4 (ms) = 1288 msec
C-SCAN
C_SCAN
100
150
160
184
18
38 39
55 58
90
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
رشیا
هرماش
C-SCAN
order of service
request serviced
track تعدادپیموده های
شده
0 100
1 150 50
2 160 10
3 184 24
4 18 166
5 38 20
6 39 1
7 55 16
8 58 3
9 90 32
FIFO
100
55 58
39
18
90
160150
38
184
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیبtr
ac
k هارشم
SSTF
100
90
58 55
39 38
18
150
160
184
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
رشیا
هرماش
SCAN
100
150160
184
90
58 55
39 38
18
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
ارشی
رهشما
C_SCAN
100
150
160
184
18
38 39
55 58
90
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7 8 9
دهی service ترتیب
ریاش
هرماش
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