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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 Recording Physics
Magnetic WriteHead Schematics: Functioning of Gap.
Magnetic Recording Physics
Remanent magnetization pattern:
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.
Magnetic Recording Physics
Writing and Reading with a Gap Head: From top to bottom: Write Current, Magnetization Pattern, Read Current.
Magnetic Recording Physics
The read current is a (deformed) derivative of the write current. The deformation results from the length of the gap.
Magnetic Recording Physics The read current is a (deformed) derivative of the write current. The deformation results from the length of the gap.
Magnetic Recording Physics
Perpendicular Recording
Uses a Probe Head. Has the potential for better magnetization retention. MEMS
Magnetic Recording Physics
Probe Device: Remanent Magnetization is in the same direction as the probe.
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.
Magnetic Recording Physics
Magneto-Resistive Effect (MR)
GMR Standard read head.
Magnetic Recording Physics
MR-Effect: Magnetic field (red) moves electron flow in the sense current (yellow) up by an angle of . The magnetoresistive material (blue) has different resistance based on the angle .
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.
Data Storage on Rigid Disks
Data Storage on Rigid Disks
Single platter or stack of platters
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
Thin magnetic coating Rotate at high speeds.
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
Data Storage on Rigid Disks
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 Storage on Rigid Disks
Data Access:
Seek
Place head over right track. Servo: Find the right track.
Rotational Delay
Used to be done with a special servo-surface on one of the platters. No servo data is embedded in the sector gaps.
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.
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.
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.
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.
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
Electronic Noise
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.
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:
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
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
Hard Drive Reliability
Stated Service Life
Design life
Expected service time of drive, usually rather short. (~ 3 years)
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
Hard Drive Reliability
Reliability Factors
Temperature (Cooling) Vibrations
Relevant if disks are put together in a rack.
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
Hard Drive Reliability
Hard Failure Modes
Mechanical Failures
Head and Head Assembly Failures
stuck bearings, actuator problems, … head crash, bad wiring, …
Media Failures Logic Board / Firmware Failures
Hard Drive Reliability
Shock Resistance
Quantum Corporation, http://www.storagereview.com/guide2000/ref/hdd/perf/qual/features.html
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
Hard Drive Reliability
SMART
SMART spec (SFF-8035i) 1996
Lists of 30 attributes
Attribute exceeding a threshold:
Disk is expected to die within 24 hours Disk is beyond design / usage lifetime
ATA-4
read error rates seek error rates
Internal attribute table is dropped Disk return OK or Not-OK
ATA-5
Adds ATA error logs and commands to run self-tests