coen 180 magnetic recording. magnetic recording physics leaves patterns of remanent magnetization on...

COEN 180

Magnetic Recording

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.

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.

Magnetic Recording Physics Three principal orientations of

magnetization with respect to a track: Longitudinal, Perpendicular, Lateral.

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.


Magnetic Write-Head Schematics:

Functioning of Gap.

Magnetic Recording Physics Remanent magnetization pattern:


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.


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


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


Perpendicular Recording Uses a Probe Head. Has the potential for better

magnetization retention. MEMS


Probe Device:

Remanent Magnetization is in the same direction as the probe.


Hard drives currently use exclusively longitudinal magnetization.

Switch to perpendicular is expected in the near future. Better retention Higher Areal

Densities. Lateral never used.


Magneto-Resistive Effect (MR) GMR Standard read head.


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


MR head directly reads the magnetic flux.

Gap head reads the changes in magnetic flux.

MR head can adjust the sense current. Better sensitivity.

Data Storage on Rigid Disks

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.

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.

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

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.

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

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.


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.

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.


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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Magnetic Codes

NRZ (top) and NRZI (below)

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

Magnetic Codes

Top to bottom:

PE

FM

MFM

Magnetic Code

Self-clocking: Transitions are never spaced out. Easy to synchronize clock to

transitions.

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.

Magnetic Codes

FM Frequency Modulation Transition in the middle of the cell

defines a one bit Absence means a zero bit.

Magnetic Codes

Top to bottom:

PE

FM

MFM

Magnetic Codes

FM still has potentially up to twice as many flux changes than bits.

Self clocking.

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.

Magnetic Codes

Top to bottom:

PE

FM

MFM

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.

Magnetic Codes

Top to bottom:

PE

FM

MFM

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.

Magnetic Codes

½(2,7) code

Data Code Word

10 0100

11 1000

000 000100

010 100100

011 001000

0010 00100100

0011 00001000

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.

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.

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.

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

Hard Drive Reliability Disk Infant Mortality

Disk drives fail at significantly higher rates during the first year.

Typical failure rate curve:


IDEMA proposal: Split MTBF rates in four different rates

0 months - 3 months 4 months – 6 months 7 months – 12 months 13 months - EODL


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

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

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


Reliability Factors Temperature (Cooling) Vibrations

Relevant if disks are put together in a rack.


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


Hard Failure Modes Mechanical Failures

stuck bearings, actuator problems, … Head and Head Assembly Failures

head crash, bad wiring, … Media Failures Logic Board / Firmware Failures


Shock Resistance

Quantum Corporation, http://www.storagereview.com/guide2000/ref/hdd/perf/qual/features.html


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

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

coen 180 magnetic recording. magnetic recording physics leaves patterns of remanent magnetization on...

Documents

magnetic recording slide

magnetic recording heads

magnetic flux

magnetic coating

gap head

surface of magnetic

probe head

rigid disks slide