Hard Drives

Hard drive features -  Winchester
  Limited number of physical sizes.
    Disk(s) - 3.5", 5.25", 1" and others.

    Magnetic media on platters.

    Constant rotational speed 3600, 7200, 10800 RPM

    Physical layout and construction.


Parsing of storage

  Cylinder, Head, Sector layout.
     Cylinder filled before platter.

  Originally addressed as chs
    Early design required BIOS and OS to be aware.

  Currently, lba and zoning.
    More flexible interface with BIOS and OS.    

  Sector layout.
    - Preamble, DATA, ECC block, gap - reported size NOT match experience.


Issues of optimal data access
  Find data on a physical device.
 
    Seek time.

    Rotational latency.

    Access time to begin reading.

    Transfer time  - access time and time to actually transfer data.


Other issues that contribute to data transfer latency.

  Propagation delay - circuits and cables.
 
  Distortion - limits distance between controller and drive.
     Ide converts analog to digital on drive.

  Attenuation - loss of signal.

  Resistance - DC. 
  Capacitance, capacitive coupling - AC.
  Impedance, inductance - AC.

  Some Solutions :
    Digital - limit distance, encoding, repeaters, Serial vs. Parallel

    Analog - Dolby (not applicable to hd).


Interleaving
  Problem
    Multi-sector not always optimally arranged.

  Solution
    Interleaving, Buffers. - track level.
    Defragmentation - drive level.


Interfaces
  Hard drives are mechanical, magnetic and analog.
  
  CPUs and memory are electronic and digital.

  An interface
    Translates (transparently) signals (levels and meanings).
    Encodes/decodes data between storage and cpu.
    Handles timing issues and handshaking.
    Buffers data.
    Address translation.
      Lies about real number of chs.


Original Encoding types
  General
    Uses small magnetic dots (domains) to record data.

    Changes in flux reversal rather than actual polarity represents data,
      easier to detect and non-arbitrary. 
    
    Allows for embedded timing, even if long strings of just 1's or 0's

  MFM -  modified FM. 
    Earliest hard drives and current floppy.
 
  RLL (and NRZI) - run length limited - 50% improvement in density.
    Next stage in pc/harddrive interface.  

  PRML (EPRML) - partial response, maximum likeliood. 30%-40% denser. > 2GB.
    Domains - smaller & closer - tend to interfer with each other.

    Read and write heads use different technology - reads smaller.

    Take several narrow samples and guesses. 

  EPRML (improved version) can offer another 20%-70% over PRML

    PRML/EPRML - data still RLL encoded and independent of ATA-IDE/SCSI


Horizontal vs. Perpendicular domains.

Lectures