CS161
Lecture 25 - Disks
Laxmi Bhuyan
http://www.cs.ucr.edu/~bhuyan/
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Magnetic Disks
Computer
Processor Memory Devices
(active) (passive) Input
Control (where
(“brain”) programs, Output
Datapath data live
(“brawn”) when
running)
Keyboard,
Mouse
Disk,
Network
Display,
Printer
°Purpose:
• Long-term, nonvolatile, inexpensive
storage for files
• Large, inexpensive, slow level in the
memory hierarchy (discuss later)
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Photo of Disk Head, Arm, Actuator
Spindle
Arm
Head
Actuator
Platters (12)
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Disk Device Terminology
Arm Head
Actuator
Inner Outer
Sector
Track Track
Platter
° Several platters, with information recorded
magnetically on both surfaces (usually)
° Bits recorded in tracks, which in turn divided into
sectors (e.g., 512 Bytes)
° Actuator moves head (end of arm,1/surface) over
track (“seek”), select surface, wait for sector rotate
under head, then read or write
•
“Cylinder”:
all
tracks
under
heads
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Disk Device Performance
Outer
Track
Platter
Inner Sector
Head Arm Controller
Spindle
Track
Actuator
°Disk Latency = Seek Time + Rotation Time
+ Transfer Time + Controller Overhead
° Seek Time? depends no. tracks move arm, seek
speed of disk
° Rotation Time? depends on speed disk rotates,
how far sector is from head
° Transfer Time? depends on data rate (bandwidth)
of disk (bit density), size of request
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Disk Device Performance
°Average distance sector from head?
°1/2 time of a rotation
• 7200 Revolutions Per Minute  120 Rev/sec
• 1 revolution = 1/120 sec  8.33 milliseconds
• 1/2 rotation (revolution)  4.16 ms
°Average no. tracks move arm?
• Sum all possible seek distances
from all possible tracks / # possible
- Assumes average seek distance is random
• Disk industry standard benchmark
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Data Rate: Inner vs. Outer Tracks
°To keep things simple, orginally kept same
number of sectors per track
• Since outer track longer, lower bits per inch
°Competition  decided to keep BPI the
same for all tracks (“constant bit density”)
 More capacity per disk
 More of sectors per track towards edge
 Since disk spins at constant speed,
outer tracks have faster data rate
°Bandwidth outer track 1.7X inner track!
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Disk Performance Model /Trends
° Capacity
+ 100%/year (2X / 1.0 yrs)
°Transfer rate (BW)
+ 40%/year (2X / 2.0 yrs)
°Rotation + Seek time
– 8%/ year (1/2 in 10 yrs)
°MB/$
> 100%/year (2X / <1.5 yrs)
Fewer chips + areal density
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State of the Art: Ultrastar 72ZX
• 73.4 GB, 3.5 inch disk
• 2¢/MB
Track
Sector
Cylinder
Track Arm
Platter
Head
Buffer
Latency =
Queuing Time +
Controller time +
per access Seek Time +
+
Rotation Time +
per byte
Size / Bandwidth
{
source: www.ibm.com;
www.pricewatch.com;
2/14/00
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• 10,000 RPM;
3 ms = 1/2 rotation
• 11 platters, 22
surfaces
• 15,110 cylinders
• 7 Gbit/sq. in. areal den
• 17 watts (idle)
• 0.1 ms controller time
• 5.3 ms avg. seek
• 50 to 29 MB/s(internal)
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Disk Performance Example
°Calculate time to read 1 sector (512B)
for UltraStar 72 using advertised
performance; sector is on outer track
Disk latency = average seek time +
average rotational delay + transfer time
+ controller overhead
= 5.3 ms + 0.5 * 1/(10000 RPM)
+ 0.5 KB / (50 MB/s) + 0.15 ms
= 5.3 ms + 0.5 /(10000 RPM/(60000ms/M))
+ 0.5 KB / (50 KB/ms) + 0.15 ms
= 5.3 + 3.0 + 0.10 + 0.15 ms = 8.55 ms
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Areal Density
°Bits recorded along a track
• Metric is Bits Per Inch (BPI)
°Number of tracks per surface
• Metric is Tracks Per Inch (TPI)
°Care about bit density per unit area
• Metric is Bits Per Square Inch
• Called Areal Density
• Areal Density = BPI x TPI
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Disk History
1989:
63 Mbit/sq. in
60,000 MBytes
1997:
1450 Mbit/sq. in
2300 MBytes
1997:
3090 Mbit/sq. in
8100 MBytes
source: New York Times, 2/23/98, page C3,
“Makers of disk drives crowd even more data into even smaller spaces”
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Areal Density
Year
Areal Density
1.7
1979
7.7
1989
63
1997
3090
2000
17100
100000
10000
1000
Areal Density
1973
100
10
1
1970
1980
1990
2000
Year
• Areal Density = BPI x TPI
• Change slope 30%/yr to 60%/yr about 1991
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1 inch disk drive!
°2000 IBM MicroDrive:
• 1.7” x 1.4” x 0.2”
• 1 GB, 3600 RPM,
5 MB/s, 15 ms seek
• Digital camera, PalmPC?
°2006 MicroDrive?
°9 GB, 50 MB/s!
• Assuming it finds a niche
in a successful product
• Assuming past trends continue
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Fallacy: Use Data Sheet “Average Seek” Time
°Manufacturers needed standard for fair
comparison (“benchmark”)
• Calculate all seeks from all tracks, divide by
number of seeks => “average”
°Real average would be based on how
data laid out on disk, where seek in real
applications, then measure performance
• Usually, tend to seek to tracks nearby, not to
random track
°Rule of Thumb: observed average seek
time is typically about 1/4 to 1/3 of quoted
seek time (i.e., 3X-4X faster)
• UltraStar 72 avg. seek: 5.3 ms  1.7 ms
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Connecting to Networks (and Other I/O)
°Bus - shared medium of
communication that can connect to
many devices
°Hierarchy of Buses in a PC
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Buses in a PC: connect a few devices
Memory
CPU
bus
Memory
PCI
Interface
°Data rates
PCI:
Internal
(Backplane)
I/O bus
Ethernet
SCSI
Interface Interface
• Memory: 133 MHz, 8 bytes
 1064 MB/s (peak)
• PCI: 33 MHz, 8 bytes wide
 264 MB/s (peak)
SCSI:
External
I/O bus
(1 to 15 disks)
Ethernet
Local
• SCSI: “Ultra3” (80 MHz),
Area
“Wide” (2 bytes)
Network
Ethernet:
 160 MB/s (peak)
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12.5 MB/s (peak)
Why Networks?
°Originally sharing I/O devices between
computers
(e.g., printers)
°Then Communicating between
computers
(e.g, file transfer protocol)
°Then Communicating between people
(e.g., email)
°Then Communicating between
networks of computers
 Internet, WWW
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Growth Rate
100,000,000
90,000,000
80,000,000
Internet Hosts
70,000,000
60,000,000
50,000,000
40,000,000
30,000,000
Ethernet Bandwidth
20,000,000
1983
3 mb/s
10,000,000
1990
10 mb/s
1997
100 mb/s
1999
1000 mb/s
0
Jan-93
Apr-95
Jun-97 Aug-99
"Source: Internet Software Consortium (http://www.isc.org/)".
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What makes networks work?
°links connecting switches to each
other and to computers or devices
Computer
switch
switch
switch
network
interface
°ability to name the components and to
route packets of information messages - from a source to a
destination
°Layering, protocols, and
encapsulation as means of abstraction
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Typical Types of Networks
°Local Area Network (Ethernet)
• Inside a building: Up to 1 km
• (peak) Data Rate: 10 Mbits/sec, 100 Mbits
/sec,1000 Mbits/sec (1.25, 12.5, 125 MBytes/s)
• Run, installed by network administrators
°Wide Area Network
• Across a continent (10km to 10000 km)
• (peak) Data Rate:
1.5 Mbits/sec to 2500 Mbits/sec
• Run, installed by telephone companies
°Wireless Networks, ...
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Network Basics: links
0110
0110
°Link made of some physical media
• wire, fiber, air
°with a transmitter (tx) on one end
• converts digital symbols to analog signals
and drives them down the link
°and a receiver (rx) on the other
• captures analog signals and converts
them back to digital signals
°tx+rx called a transceiver
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Example: Network Media
Twisted Pair:
Copper, 1mm think, twisted to
avoid antenna effect
Used by cable
Coaxial Cable:
companies: high
Plastic Covering
Braided outer conductor BW, good noise
Insulator
immunity
Copper core
Light:
3 parts are
Total internal
Fiber Optics
cable, light
reflection
Transmitter
Air
source,
– L.E.D
– Laser Diode
Receiver
light
– Photodiode
light
detector
source
Silica
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ABCs of Networks: 2 Computers
° Starting Point: Send bits between 2 computers
appln
appln
OS
network
interface
° Queue
device
OS
(First In First Out) on each end
° Can send both ways (“Full Duplex”)
° Information sent called a “message”
• Note: Messages also called packets
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ABCs: many computers
appln
appln
OS
OS
network
interface
device
°switches and routers interpret the
header in order to deliver the packet
°source encodes and destination
decodes content of the payload
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