Secondary storage access methods.

  Random access to sequential.
    
    True random access - byte level addressing.
      Impractical - requires technology to access storage at byte level like 
      RAM memory.

      Requires method to store and reference all addresses.  The data for 
      tracking the data would be more than the data.      

    Sequential - read bytes in sequence with loop mechanism.
      Just need to find start of block of data and specify length.

      Very cheap to implement.

      Tends to be slow.
  
      Useful when working with very large data sets such has disk backup.
         Tape is still in use as inexpensive, transportable backup.

      Or sequentially reading a file - streaming a movie.

      Keyboard - user randomly strikes keys, but OS is sequentially
        reading the keys chosen.

  Random access hybrid - Conceptually
    Picture a set of file cabinets

    You can directly go to the cabinet and drawer of interest.

    You can skip search to the needed folder.

    Once you have the folder, you usually have to perform a slower
      sequential browse of each page in the folder until you find the
      data of interest.

    Solid State Drives 
      + Throughput up to 2.5 GB/s possible.
      + SSDs fairly good physical tolerances.
      - Use fatigue
      - Logic more complex and expensive. 
      - Fair amount of meta-data (data about where data is).
      + Low power
      https://en.wikipedia.org/wiki/Solid-state_drive 

    Hard drives 
      + With SATA interface, 200 MB/s throughput.
      - Costs cheaper than SSD but more expensive than sequential storage.
      - Systems are more fragile.
      - Fair amount of meta-data (data about where data is).
      - Technology often dissimilar to CPU/motherboard,
          requiring additional logic.

    Optical drives (CD, DVD, Blu-Ray) 
      - More serial that random
      - poor random/random block access
      + good streaming access, movies, music, etc.
      - 54 MB/s throughput Blu Ray (data)
      # Note that most audio/video streams well under the maximum 
        throughput possible.

  Mechanisms for addressing storage :

    By a complex hardware specific address on storage.

      Cylinder/head/sector for hard drives.

      Logical Block Address (LBA) for hard drives and optical drives.

      Distance from start of tape and start pattern, or logical block 
        address. Tapes, CD, DVD, Blu-ray.

    Symbolically (by file names) 
      Potential quantity of "data sets" requires a friendlier mode of 
        addressing. Use "filename" and translate into addressing mode native 
        to storage device.

      Referred to as meta-data (data about data).

      Uses look-up tables to convert symbolic to hardware specific addresses.

    Interfacing external hardware to system.
      Through dedicated registers and ports or mapped addresses in main memory.
      Speed differences require special routines and interrupts
        to avoid CPU waiting long times.

      Device response may very greatly depending on technology.
        Hard drive - read/write
        Optical drive - read/write
        Printers - primarily write only
        Keyboard - primarily read only
        Monitor - primarily write only.

  Requiring both software and hardware support.

  Access is much slower than primary memory (> 1000 x Slower).
    Primarily because I/O tends to be at least partially mechanical.

  Much cheaper than primary memory.
  

Types of secondary memory - favors random
  Solid State drives (SSD) - High-end flash memory.
    Very fast - 6Gib/sec or 500MiB/sec
      depends on bus interface.
    Most expensive commercial 2ndary storage. 
    Non-mechanical. very fast. 
    Internally, accessed sequentially at the block and page level 
       using symbolic markers.  
    Page 512 bytes, 2K, 4K, 8K.
      Page directly addressed - contents read sequentially.
    Fairly rugged but expensive.
    Available in Hard drive sizes at about 10x cost.

    ~ 45-60¢/GB  Spring 2015
    ~ 30-40¢/GB  Spring 2016  120GB-500GB
    ~ 25-35¢/GB  Spring 2018  120GB-500GB
    ~ 17-22¢/GB  Spring 2018  500GB

    * write wear shortens lifespan. 
      Set up swap on alternative drive if possible.

    SSD with PCIe interface 500GB $200 - 3.2GB/s read / 1.9 GB/s write.
       V-NAND (stacked cells)  (2018 Spring)

  Memory sticks - same technology, smaller amounts (64MB-16GB), 
    Quality not as good keeps pricing down. 
    Externally mounted and accessed via USB sequentially, so slower than SSDs.
    Page 512 bytes, 2K, 4K, 8K.
      Page directly addressed - contents read sequentially.
    * USB 2 - 480MBit/sec. (~ 60 MB/s), USB 3 - 5 Gbit/sec. (~600 MB/s).
    ~ ~ 32¢/GB - 8, 16, 32, 64 GB, 256 GB common.
  
    Note : older OSes have trouble reading anything larger than 8 GB.

  Hard drives - fairly expensive storage, but fast, extremely large 
    capacities (3 TB common), highly reusable, fragile.  
    Direct access at the Cylinder/Head level.
    Sequential at the track->sector/cluster level.
    512 byte or 4K sectors are standard.
    Additionally, sectors often grouped into clusters or blocks to simplify
      and expand addressing range.
    ~ < 5¢/GB Spring 2015
    ~ < 3¢-6¢/GB Spring 2016  1TB to 2TB common, larger available.
    ~ < 3¢-5¢/GB Spring 2018  1TB to 2TB common, larger available.
    ~ 3 TB common in Fall 2018
    # cost of physical drive greatest expense, 
       so 1TB, 2TB, and 3TB often same cost.

  CD/DVD/Blueray  - cheap, slower than hard drives, smaller storage capacity,
    Less flexible (re-usage limited).

    Good resilience. Requires some care, but easily transportable.

    Primary design is for long sequential (streaming) reads of audio/video
      data
     
    Requires skip sampling to find sector of interest.

    Sequential at the sector/cluster level, aprox 2K sectors.

    Read only, WORM(burnable), limited r/w
      CD ~ < 16¢/disc (~700 MiB) - close to stable price
         - mimimum manufacturing cost. 
         ~ 20-30¢/disc - Spring 2018 
   
      DVD ~ 20¢ - 1/disc (4.7 GiB) 
      BluRay ~ < ¢70/disc SL (25GiB)
      BluRay ~ < $1.25/disc DL (50 GiB)

  CD 150 KB/s audio, 52x 7.8 MB/s data, some faster.

  DVD 1.32 MB/s video, 16x 21 MB/s
    en.wikipedia.org/wiki/CD_and_DVD_writing_speed

  Blu-ray 4.5 MB/s 1x, 9 MB/s 2x (movies), 72 MB/s 16x
    https://en.wikipedia.org/wiki/Blu-ray_Disc_recordable

  Disc rotation seems to be limited to 10K rpm.
    Possible safe structural limit of poly-carbonate disc.

  Improvement from bits closer together and improved
    (more efficient) encoding.


  Other removable disk media (zip, floppies, etc) - cheaper, slower,
    very flexible, small capacity, resilience varies. Random/sequential
    access similar to hard drives.

    Most of these have been replaced by cd/dvd, memory sticks, external
      hard drives using USB interface, or networked external storage.

Types of secondary memory - highly sequential
  Magnetic tape - very cheap, very slow (linear access), medium storage
    capacity, limited flexibility, better than hard drive.

    ~$20/1.5TB  Fall 2018
    * drive ~ $1000
      * compression can double storage but slows process 

      Check out : www.gizmag.com/sony-185-tb-magnetic-tape-storage/31910/

    Sequential access similar to CD drives. 
    * Lately, reliability of tape for backup questioned. 
    * Then again, maybe not :
    *https://www.infostor.com/backup-and_recovery/tape/10-reasons-tape-storage-is-better-than-disk.html
    
  Punch (tape or card) - very cheap, extremely slow, low storage
    capacity, limited flexibility, extremely resilient.

    Obsolete.