Lecture #39: Magnetic memory storage
• Last lecture:
– Dynamic Ram
– E2 memory
• This lecture:
– Future memory technologies
– Magnetic memory devices
– Hard drives, tape drives, Optical disks
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Future memory technologies
• Memory speed, cost and density are
among the chief bottlenecks on compute
power.
• Increasing CPU clock rates have only
resulted in small increases in speed of
operation due to the memory system and
mass storage (disk) I/O bottleneck.
• A significant amount of research effort is
directed to improving memory technology
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Advanced memory technologies
• Ferroelectric Random Access Memory (FRAMs)
• Magnetoresistive Random Access Memories
(MRAMs)
– Tunneling Magnetic Junction RAM (TMJ-RAM):
• Experimental Memories
– Quantum-Mechanical Switch Memories
– Single Electron Memory
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FRAM
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Ferroelectric material
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TMJ-Ram
• Tunneling Magnetic Junction RAM (TMJ-RAM):
– Speed of SRAM, density of DRAM, nonvolatile (no refresh)
– “Spintronics” (electron spin affects transport)
– Same technology used in the read heads of
high-density disk-drives: Giant magnetoresistive effect
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Tunneling Magnetic Junction
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Mass Storage
• For storage of larger amounts of
information, magnetic film storage
dominates
• Information is stored in the form of
magnetic domains in a Ferromagnetic film,
written or read by a moving head
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Magnetic domains
• Ferromagnetic materials have a quantum
interaction which makes adjacent atoms
line up their magnetic field in the same
direction
N N N N N N N N N N N N N
S S S S S S S S S S S S S
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Magnetic interactions
• On a larger scale, magnets feel a force to line up in
opposing directions, reducing the total magnetic field.
• For example, if you try to hold two magnets next to each
other, there will be a strong force which will rotate them
to the configuration:
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N
S
S
N
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Magnetic domains
• If you look microscopically at a magnetic material, it forms domains,
or areas where the magnetic poles are aligned, adjacent to regions
where the magnetization is in the opposite direction.
• In a thin film, the domains look like this:
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Moving magnetic domains
• Magnetic domains don’t move easily at room
temperature, but they can be changed by
applying magnetic fields.
• If most of the domains in a material are aligned
in one direction, we call it a permanent magnet.
• The core of an inductor or a transformer is made
of a ferromagnetic material where the domains
line up easily, and then randomize again when
the external field is turned off
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Writing to magnetic media
• Magnetic storage material is comprised of
a thin film of ferromagnetic material which
is relatively magnetically hard.
• A small electromagnet is used to create
domains oriented in a particular direction
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Reading magnetic material
• Conventional read heads for magnetic media
work just like the secondary winding of a
transformer.
• Instead of a primary winding changing the
magnetic field through a coil, and thus changing
the voltage, the magnetic media is moved next
to the read coil.
• This produces a voltage across the read coil
which can be amplified and translated as data
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Transformer
+
+
Transformer
V1
V2
-
-
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V1 N1

V2 N 2
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Storage density for DRAM vs DISK
9 v. 22 Mb/si
45%
40%
35%
30%
25%
20%
15%
470 v. 3000 Mb/si
10%
5% 0.2 v. 1.7 Mb/si
0%
1974
1980
1986
1992
1998
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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SRAM vs. DRAM vs. Disk
– Access latencies:
• DRAM ~10X slower than SRAM
– Successive bytes 4x faster than first byte for DRAM
• Disk ~100,000X slower than DRAM
– First byte is ~100,000X slower than successive bytes on
disk
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Nano-layered Disk Heads
• Recent large improvement in Disk capacity comes from
“Giant Magneto-Resistive effect” (GMR) read heads
Coil for writing
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Typical Numbers of a Magnetic Disk
Track
Sector
• Rotational Latency:
– Most disks rotate at 3,600 to 15,000 RPM
– Approximately 16 ms to 4 ms
per revolution, respectively
– An average latency to the desired
information is halfway around the disk:
8 ms at 3600 RPM, 2 ms at 15,000 RPM
Cylinder
Head
Platter
• Transfer Time is a function of :
–
–
–
–
–
Transfer size (usually a sector): 1 KB / sector
Rotation speed: 3600 RPM to 10000 RPM
Recording density: bits per inch on a track
Diameter typical diameter ranges from 2.5 to 5.25 in
Typical values: 2 to 80 MB per second
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Disk Device Terminology
Arm Head
Inner Outer
Sector
Track Track
Platter
Actuator
• 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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Photo of Disk Head, Arm, Actuator
Spindle
Arm
Head
Actuator
Platters (12)
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Disk Device Performance
Outer
Track
Inner Sector
Head Arm Controller
Spindle
Track
Platter
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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Devices: Magnetic Disks
• Purpose:
Track
Sector
– Long-term, nonvolatile storage
– Large, inexpensive, slow level in
the storage hierarchy
• Characteristics:
Cylinder
– Seek Time (~8 ms avg)
•
•
•
7200 RPM = 120 RPS => 8 ms per rev
ave rot. latency = 4 ms
128 sectors per track => 0.25 ms per sector
1 KB per sector => 16 MB / s
Transfer rate
–
–
10-30 MByte/sec
Blocks
• Capacity
–
–
Gigabytes
Quadruples every 3 years
(aerodynamics)
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Head
positional latency
rotational latency
Platter
Response time
= Queue + Controller + Seek + Rot + Xfer
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Service time
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Areal Density
1973
1979
1989
1997
2000
2004
Areal Density
1.7 1000000
7.7
100000
63
3090
10000
17100
1000
130000
Areal Density
Year
100
10
1
1970
1980
1990
2000
2010
Year
–Bits per unit area changed slope from 30%/yr
to 60%/yr about 1991
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Technology Trends
Disk Capacity
now doubles
every
12 months; before
1990 every 36 motnhs
• Today: Processing Power Doubles Every 18 months
• Today: Memory Size Doubles Every 18-24 months(4X/3yr)
The I/O
GAP
• Today: Disk Capacity Doubles Every 12-18 months
• Disk Positioning Rate (Seek + Rotate) Doubles Every Ten Years!
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