Hard Disks

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Hard Disks
Top view of a 36 GB, 10,000 RPM, IBM SCSIserver
hard disk, with its top cover removed.Note the
height of the drive and the 10 stacked
platters.(IBM Ultrastar 36ZX.)
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Hard Disk Basics

• Invented in the 1950
• 20 inch diameter holding a few megabytes
• Originally called fixed disks or Winchesters
  (Winchester 30-30 rifle)
• Magnetic recording technique
• Easily erased and rewritten
• Remembers magnetic flux patterns for many years.

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Hard Disk Basics

• hard disks play a significant role in the
  following important aspects of a computer system
• Overall System Performance
• Storage Capacity
• Software Support
• Reliability

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Hard Disk Trends

• Areal Density 35Gbits/inch2 (20GB on a single
  3.5 platter)
• Capacity 300GB
• Spindle speed 15000rpm
• Form factor smaller -gt more rigid
• Desktop 5.25 -gt 3,5
• Laptop 2,5
• IBM Microdrive 1 diameter, 0,25 thick, 340 MB
• Performance positioning and transfer performance
• Reliability (SMART)
• RAID for performance reliability
• Interfaces IDE/SATA SCSI/SAS - USB

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Construction
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Platters and Media

• One or more platters
• Substrate, gives it structure and rigidity
• Magnetic media coating
• Platters are precision manufactured
• Assembled in a clean room to keep platters clean.

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Platter Size Drive form factor

• Form factor 5.25,3.5,2.5, PC Card
  CompactFlash

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Platter Size is shrinking, why ?

• Most cost effective for manufacturers platter
  size as big as possible !
• Reasons for shrinking
• Enhanced rigidity more rsistant to shock and
  vibration. (diameter /2 -gt rigidity x 4)
• Manufacturing Ease
• Mass Reduction (easier to spin, faster spin-up)
• Power conservation
• Noise and Heat reduction
• Improved Seek Performance, but smaller capacity !

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Platter size shrinking

• Seagate Cheetah 10K

• Seagate Cheetah 15K

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Number of Platters

• Consumer disks 1 5 platters
• High-end disks up to 12 platters
• Always a single assembly that spins as one unit.
• Each platter two surfaces to hold data -gt two
  read/write heads. (not always used)
• Many platters difficult to drive, noise,
  vibration, spin up, spin down, ...
• Trend less platters (areal density compensates)
• Height limitation one inch for standard drives.

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Platter Substrate Materials
Material should be rigid, easy to work with,
lightweight, stable, magnetically inert,
inexpensive, available.

• Aluminium Alloy Platter

• Glass platter

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Glass Platters advantages over aluminium

• Better quality smoother surface, ideal for low
  flying heights and faster spindle speeds.
• Improved rigidity glass is more rigid than
  aluminium
• Thinner platters enhanced rigidity allows
  platters to be made thinner. More platters can be
  placed in the same drive dimensions. Thinner
  platters weigh less gt spindle motor requirements
  reduced, spin-up time smaller
• Thermal Stability glass expands less than
  aluminium when heated.
• Very thin glass FRAGILITY
• Solution Glass/ceramic composites to reduce the
  likelyhood of cracking.

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Magnetic Media

• Thin coating of magnetic material (only a few
  millionths of an inch thick)
• Older disks used high-performance oxide media
  (rust particles), similar to what is used in
  audio cassette tape.
• Oxide media is inexpensive but
• Is soft, easily damaged
• Low density
• Todays disks use thin film media
• Much more uniform and smooth
• Superior magnetic properties (higher areal
  density)
• Harder and more durable than oxide

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Tracks and Sectors

• Platter is broken in tracks
• Each track divided into sectors.
• Sector is smallest accessable unit which holds
  512 bytes of info.
• First disks 17 sectors per track
• Today over 1000 sectors per track zoned
  recording

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Zoned Recording

• 5400RPM IBM 40GV (20GB)

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Interleaving

• Used only for (Old) slow controllers
• Interleave factor 11
• 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17
• Interleave factor 21
• 1,10,2,11,3,12,4,13,5,14,6,15,7,16,8,17,9
• Interleave factor 31
• 1,7,13,2,8,14,3,9,15,4,10,16,5,11,17,6,12
• Configured by low level format

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Cylinder and Head Skew
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Magnetic Reading Recording

• MR (Magneto-Resistive) or GMR (Giant MR) reading
  head.
• Thin-Film inductive write head

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Ferrite Head

• Mid 1970s
• Bulky ferrite head

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Thin Film Heads (TF)

• 1980s to mid 1990s
• Very small, precise head
• Much higher density (100-1000MB drives)

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Magnetoresistive (MR) Heads

• Ferrite and Thin Film heads inducing a current
  in the wire of the read head when a magnetic
  field is present.
• MR heads (reading only) special conductive
  material that changes resistance when a magnetic
  field is present. A sensor detects these changes
  in resistance. Results in much higher densities
  on the platters. Read head is much more sensitive
  -gt allows weaker written signals, which lets the
  bits be spaced closer together.
• 1GB 30GB disks

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Giant Magnetoresistive (GMR) Heads

• Same principle as MR heads
• Different design that makes it superior
• Giant MR head is smaller than MR head
• Giant magnetoresistive effect discovered in the
  late 1980s
• Thin layers of various magnetic materials
• Subjected to magnetic field
• gt large resistance changes
• Commercialized by IBM GMR
• First harddisk using GMR on the market 1998

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Giant Magnetoresistive (GMR) Heads

• GMR head operation

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MR versus GMR

• GMR is more sensitive
• MR only 2 resistance change when polarity
  changes
• GMR from 5 to 8
• GMR head can detect much weaker (smaller)
  signals. Much higher areal density is possible.
• GMR head 300GB disks (35Gbits/in2)

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Illustration inside a hard disk

• Speed 100km/h

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Sources

• Harddisks http//www.pcguide.com/ref/hdd
• Seagate Technology Papers http//www.seagate.com/
  newsinfo/newsroom/papers/index.html
• Seagate Technology More than an Interface SCSI
  vs. ATA Dave Anderson, Jim Dykes, Erik
  Riedel
• Hitachi Global Storage Technologies
  http//www.hitachigst.com/hdd/hddpdf/tech/hdd_tech
  nology2003.pdf