Magnetic recording leaves patterns of magnetization on magnetic media to store data. Tracks are formed as the read/write head passes over the media. There are three main orientations for magnetization: longitudinal, perpendicular, and lateral. Longitudinal recording uses a ring-shaped electromagnet head with a gap to magnetize the media as it moves under the head. Changes in the current passing through the head leave spatial variations in magnetization along the track. Modern drives use magneto-resistive read heads that directly sense the magnetic flux from the tracks. Error correcting codes and redundant data help ensure reliable storage despite defects and noise sources that can cause errors.

1. COEN 180
Magnetic Recording

2. Magnetic Recording Physics
 Leaves patterns of
remanent
magnetization on a
track within the
surface of magnetic
media that sits on
top of a physical
substrate.

3. Magnetic Recording Physics
 Track formed by head passing over it.
 We say that the head flies over the
track, i.e. we assume the view point of
the head.

4. Magnetic Recording Physics
 Three principal orientations of
magnetization with respect to a track:
 Longitudinal, Perpendicular, Lateral.

5. Magnetic Recording Physics
 Longitudinal recording:
 Transducer is ring-shaped electromagnet with a
gap at the surface facing the media.
 If head is fed with current, the fringing field from
the gap magnetizes the magnetic media.
 Media moves at constant velocity under the head.
 Temporal changes in the current leave spatial
variations in the remanent magnetization along the
length of the track.

6. Magnetic Recording Physics
Magnetic Write-
Head Schematics:
Functioning of Gap.

7. Magnetic Recording Physics
 Remanent magnetization pattern:

8. Magnetic Recording Physics
 Read head used to be the same as
write head.
 Passing the gap head over the track
would let the magnetization pattern
cause an induced read current.

9. Magnetic Recording Physics
Writing and Reading with a Gap Head: From top to bottom: Write Current,
Magnetization Pattern, Read Current.

10. Magnetic Recording Physics
The read current is a (deformed) derivative of the write current. The deformation
results from the length of the gap.

11. Magnetic Recording Physics
The read current is a (deformed)
derivative of the write current.
The deformation results from the
length of the gap.

12. Magnetic Recording Physics
 Perpendicular Recording
 Uses a Probe Head.
 Has the potential for better magnetization
retention.
 MEMS

13. Magnetic Recording Physics
Probe Device:
Remanent Magnetization is
in the same direction as the
probe.

14. Magnetic Recording Physics
 Hard drives currently use exclusively
longitudinal magnetization.
 Switch to perpendicular is expected in
the near future.
 Better retention  Higher Areal Densities.
 Lateral never used.

15. Magnetic Recording Physics
 Magneto-Resistive Effect (MR)
 GMR
 Standard read head.

16. Magnetic Recording Physics
MR-Effect: Magnetic field (red) moves electron flow in the
sense current (yellow) up by an angle of θ. The magneto-
resistive material (blue) has different resistance based on
the angle θ.

17. Magnetic Recording Physics
 MR head directly reads the magnetic
flux.
 Gap head reads the changes in
magnetic flux.
 MR head can adjust the sense current.
 Better sensitivity.

18. Data Storage on Rigid Disks

19. Data Storage on Rigid Disks
 Single platter or stack of platters
 Thin magnetic coating
 Rotate at high speeds.
 Magnetic recording heads mounted on arms record
data on all surfaces.
 Heads moved across the disk surface by a high speed
actuator.
 Circular tracks.
 Cylinder
 Formed by the tracks on all surfaces by same actuator
position.
 The tracks are broken up into sectors (or disk blocks).
 The old format of 512B per block still remains in effect.

20. Data Storage on Rigid Disks

21. Data Storage on Rigid Disks

22. Data Storage on Rigid Disks
 Hard drives rotate at constant angular
speed.
 Constant linear velocity impractical.
 Heads see more track in the outer layers.
 Nr. of sectors per track varies.
 Remains constant in a “band”.
 Data density increases in a band as we
move to the inside.

23. Data Storage on Rigid Disks
 The platter consists of a rigid aluminium
or glass platter, coated with various
coats.
 Rigid platter
 Magnetizable thin film that actually stores the
data.
 Overcoat
 Lubricant

Protects (somewhat) against head crashes

24. Data Storage on Rigid Disks
 Use surrounding air pressure to maintain the
proper distance between head and the surface
 The spacing controls the focus of the head; if the
head is further away from the surface, then it will read
from and write to a wider area.
 To increase data densities, the head - surface spacing
has decreased dramatically.
 The head can no longer be parked on the surface
during power down (when the rotation ceases, the
head will actually land).
 Special landing area.

Surface is treated to allow air to get between the head
and the surface.

When head flies again, move over the data tracks.

25. Data Storage on Rigid Disks

26. Data Storage on Rigid Disks
 Data Access:
 Seek

Place head over right track.

Servo: Find the right track.
 Used to be done with a special servo-surface on
one of the platters.
 No servo data is embedded in the sector gaps.
 Rotational Delay

On average half the time of a disk revolution.

AKA latency.
 Transfer Time

27. Data Storage on Rigid Disks
 Performance Parameters:
 Capacity / Data Density

Disks with smaller form factors have become
popular in niche applications.

Trend towards smaller disk, that can rotate faster.
 Data density is a two-dimensional value:

tpi: Tracks per inch: How far do tracks have to be
separated.

bpi: bits per inch: How many sectors on a single
track.

28. Data Storage on Rigid Disks
 Operations on adjacent
tracks can interfere with
each other:
 Track misregistration.
 During read

Too much noise.
 During write

Data written can be
unreadable.

Data on next track can
become unreadable.

29. Data Storage on Rigid Disks
 Data Density:
 Limited by the ability to distinguish distinct
magnetization patterns.
 Pulse superimposition theory:

Flux from nearby magnetization patterns
influences reads.

30. Data Storage on Rigid Disks
Read current picked up by a
magnetic gap head.
Red line: Read current in absence of the
other change.
Green line: Resulting read current.
Top: No interference.
Middle: Peak shifts to the outside.
Bottom: Peak shift much more
pronounced.

31. Data Storage on Rigid Disks
 Seek time:
 Determined by the speed of the actuator.
 Determined by the capacity of the servo
mechanism.
 If the actuator moves very fast, then there is more
of a settling time.

32. Data Storage on Rigid Disks
 Latency:
 Solely determined by rotational speed.
 Rotational speed limited by the
aerodynamics of the platter.
 Larger platters cannot be rotated as fast as
smaller ones.

33. Data Storage on Rigid Disks
 Access Time:
 Random Access

Seek

Latency

Transfer
 Stream (block after block)

Only first seek, only first latency.

Zero Latency Disk
 Starts reading whenever data needed appears under the
head.
 Others wait for the first block of the stream.

Occasional track to neighboring track seeks.

34. Data Storage on Rigid Disks
 Errors
 Disks are not intended for error-free
operations.
 Soft error

Error cannot be repeated.
 Hard error

Cannot do the operation.

35. Data Storage on Rigid Disks
 Interference
 Cross-talk between different channels or
through feedthrough.
 Track Misregistration.
 Imperfect Overwrites / Incomplete Erasures.
 Side fringing

when the head picks up flux changes from an
adjacent track.
 Bit loss due to Intersymbol Interference.

36. Data Storage on Rigid Disks
 Noise
 Media noise

Defects or random media properties
 Spot on the surface does not retain magnetization because of a
manufacturing problem or because of a previous head crash.
 A modern disk drive has spare sectors on each track and
complete spare tracks to substitute for sectors that have these
defects.
 Even without an outright defect, the magnetic properties of the
medium vary.
 Electronic Noise

caused by random fluctuations typically in the first stage
amplifier in the reproducing circuit.
 Head Noise:

The magnetic flux in both write and read heads is subject to
thermally induced fluctuations in time.

37. Data Storage on Rigid Disks
 Error rate is controlled through the use of
Error Control Codes.
 In addition, each sector has a checksum
to prevent false data from being read.

38. Data Storage on Rigid Disks
 Reliability
 Device failure

SMART (UCSD MRC) can predict 50% failures
based on higher rate of soft errors.
 Block failure: bit rot
 Data corruption: bit rot that is undetected.

39. Data Storage on Rigid Disks
 Power Use
 Major problems for laptops.
 Major problems for very large disk-based
storage centers.
 Various proposals of spinning up / down
strategies:

MAID: Massive Arrays of Idle Disks.
 System Interface:
 SCSI vs. IDE.

40. Magnetic Codes
 Magnetic codes bind the bit stream to
magnetization patterns.
 Direction of write current determines the
direction of magnetization
 Easiest: NRZ code

No Return to Zero Code.

Needs clocking.

41. Magnetic Codes
 NRZ Code: Vertical lines are clock ticks.
 They define a window.
 Write current in one direction is a zero, in other is a
one bit.
 We detect magnetization changes (Peak detection).
 We miss one, we reverse the rest of the string.

42. Magnetic Codes
 NRZI
 No Return on Zero Inverted
 Switch magnetization pattern = 1
 No switch during window = 0.
 Has difficulties of counting with long strings
of zeroes.

43. Magnetic Codes
NRZ (top) and NRZI (below)

44. Magnetic Codes
 Phase encoding:
 Transition up for a one in window
 Transition down for a zero in window
 Two or more zeroes / ones in a row:

Additional transition in the middle.
 Self-clocking

45. Magnetic Codes
Top to bottom:
PE
FM
MFM

46. Magnetic Code
 Self-clocking:
 Transitions are never spaced out.
 Easy to synchronize clock to transitions.

47. Magnetic Codes
 Problem with PM:
 Up to twice as many flux changes than
transitions.
 Limits bit density because flux changes too
close together leads to noisy signal.

48. Magnetic Codes
 FM
 Frequency Modulation
 Transition in the middle of the cell defines
a one bit
 Absence means a zero bit.

49. Magnetic Codes
Top to bottom:
PE
FM
MFM

50. Magnetic Codes
 FM still has potentially up to twice as
many flux changes than bits.
 Self clocking.

51. Magnetic Codes
 MFM
 Delay Modulation / Miller Code
 Transition in the middle of the cell for a one.
 No transition in the middle of the cell for a zero bit.
 Additional transition on the window boundary
between two zeroes.
 Number of flux changes equals the number of bits.

52. Magnetic Codes
Top to bottom:
PE
FM
MFM

53. Magnetic Codes
 Generate MFM by a state
diagram.
 Data bits determine transition.
 Bits in state our output when
state is reached.

First bit for the clock window.

Second bit for the transition /
lack of transition within the
window.

54. Magnetic Codes
Top to bottom:
PE
FM
MFM

55. Magnetic Codes
 Modulation Codes
 Transform data bit string into a magnetic code.
 Written on magnetic medium as an NRZI waveform.
 3 Parameters:

d = minimum of zeroes between consecutive ones.

k = maximum of zeroes between consecutive ones.

Data density: ratio of x data bits over y magnetic code bits.
 Important for capacity:

Large values of d are important for data density:
 Flux transitions are spaced out.

Lower values of k indicate ease of synchronizing clocks.

56. Magnetic Codes
 ½(2,7) code
Data Code Word
10 0100
11 1000
000 000100
010 100100
011 001000
0010 00100100
0011 00001000

57. Magnetic Codes
 PRML channel
 Uses maximum likelihood decoding (ML)
 Partial response:

Readback pulses from adjacent transitions are allowed
to interfere with each other.

ML decoding unravels the results of interference.
 Write Precompensation
 Predistorting the write data before they are sent to
write driver

transitions are correctly placed when read.

58. Disk Defects
 Channel impairments
 Intersymbol interference
 Off-track interference
 Amplifier noise
 Disk defects

Random noise associated with the random
nature of the disk surface without defects.

Media defect.

59. Error Correcting Code
 Disks use error detection and error
correction
 Reed Solomon code example:

38 bytes added to 512 data field

Probability of uncorrectable error moves from
10-7
per bit to 8.8*10-16
.

60. Hard Drive Reliability
 Measured in Mean Time Between
Failure
 Typically quoted at > 106
hours
 Gives the probability of failure during the
economic lifespan of disk, not expected
life span.

Note: Data is expected to survive centuries

61. Hard Drive Reliability
 Disk Infant Mortality
 Disk drives fail at significantly higher rates during the first
year.
 Typical failure rate curve:

62. Hard Drive Reliability
 IDEMA proposal:
 Split MTBF rates in four different rates

0 months - 3 months

4 months – 6 months

7 months – 12 months

13 months - EODL

63. Hard Drive Reliability
 Disk Infant Mortality becomes
noticeable for management when
setting up redundancy strategies for
very large arrays of drives.
 Either:
 Increase redundancy of data stored
partially on young drives.
 Use additional burn-in times

64. Hard Drive Reliability
 Stated Service Life
 Expected service time of drive, usually rather
short. (~ 3 years)
 Design life
 Time span that a disk drive should be functioning
reliably.
 Because of technical obsolescence (performance,
capacity) < 7 years.
 Warranty Length

65. Hard Drive Reliability
 Reliability Factors
 Start / Stop Rates

Spinning down disk creates reliability problems.
 Counter measures:

Special “Landing zones” (Desktop)

Ramping (Laptop)
 Power On / Off cycles
 Air pressure

Air cushion is needed to place head at correct
distance

66. Hard Drive Reliability
 Reliability Factors
 Temperature (Cooling)
 Vibrations

Relevant if disks are put together in a rack.

67. Hard Drive Reliability
 Bad Batch Problem
 Anecdotes of “bad batches”
 Tend to show up in the first year
 But not fast enough to be caught by
quality.
 Usually dealt with silently through the
warranty process

68. Hard Drive Reliability
 Hard Failure Modes
 Mechanical Failures

stuck bearings, actuator problems, …
 Head and Head Assembly Failures

head crash, bad wiring, …
 Media Failures
 Logic Board / Firmware Failures

69. Hard Drive Reliability
 Shock Resistance
Quantum Corporation,
http://www.storagereview.com/guide2000/ref/hdd/perf/qual/features.html

70. Hard Drive Reliability
 SMART
 (Self-Monitoring Analysis and Reporting
Technology )
 Many hard errors are predictable

30% current implementations

40% - 60% with advanced decision making
Get smartctl for linux at smartmontools.sourceforge.net

71. Hard Drive Reliability
 SMART
 SMART spec (SFF-8035i) 1996

Lists of 30 attributes
 read error rates
 seek error rates

Attribute exceeding a threshold:
 Disk is expected to die within 24 hours
 Disk is beyond design / usage lifetime
 ATA-4

Internal attribute table is dropped

Disk return OK or Not-OK
 ATA-5

Adds ATA error logs and commands to run self-tests