Clemson University -- CPSC 231 -- Spring 2010


I/O - input/output

  system components: CPU, memory, and bus -- now add I/O controllers and
      peripheral devices

            +-----+
            | CPU |
            +-----+
            |cache|
            +-----+
               |
   +============================================+ bus
       |                |                 |
   +--------+     +-----------+     +-----------+
   | memory |     |controller |     |controller |
   |        |     +-----------+     +-----------+
   |+------+|           |                 |     
   || I/O  ||     +-----------+     +-----------+     
   ||buffer||     |  device   |     |  device   |     
   |+------+|     +-----------+     +-----------+
   |        |
   |        |
   +--------+


Devices

  source: e.g., keyboard, mouse, scanner
  sink: e.g., monitor, printer
  source/sink: e.g., modem, network connection
  slow memory: e.g., disk, tape

  keyboard - e.g., consider how keyboard on PC works:
    - each key press causes an interrupt and sends a scan code
    - each key release also causes an interrupt and sends a scan code
    - keyboard ISR must keep track of scan codes and translate to ASCII (e.g.,
          consider that a capital 'A' requires four interrupts)
    - auto-repeat function requires a timer to be set at each depress and
          special processing should it go off prior to key release
    - "raw mode" - all characters sent to buffer and on to program
    - "cooked mode" - special characters processed in buffer before sending
          to program - e.g., backspace and/or delete

  monitor - again, consider a PC
    - memory-mapped display buffer
    - monitor adapter refreshes display from display buffer, will often use
          special dual-ported memory chips (VRAM)

  disk
    - rotating platters covered with magnetic oxide coating
    - collection of read/write heads, one per surface, mounted on "arm", which
          moves in and out
    - concentric circles defined by where r/w head can be positioned are called
          tracks, and the set of parallel tracks defined by one arm position
          is called a cylinder
    - simple disks have fixed number of sectors per track, and small number of
          bytes per sector (e.g., 512)
    - disk access requires:
        * "seek" - arm motion of 2-20 msecs
        * "rotational latency" - wait until proper sector rotates under the
              r/w/ head, 1/2 rotation on avg., 8.3 msecs for 3600 rpm
        * "transfer" - time to read or write sector, depends on sector size,
              fraction of msec
    e.g., disk specs - Fujitsu MAP 3147
        * formatted capacity: 147 GB
          + number of disks (i.e., platters): 4
          + number of data heads: 8
          + sector size: 512 bytes
          + number of recording zones per surface: 18
            - sectors/track ranges from 533 inner zone to 936 outer zone
          + recording density (maximum): 600,000 BPI
          + track density: 63,100 TPI
            - 48,000 cylinders
        * seek time (read): 4.5 ms (avg.); track to track 0.3 ms; full 10 ms
          + write seeks are slightly longer (5ms, 0.5ms, 11ms, respectively)
        * rotational latency: 2.99 ms (avg.)
          + rotational speed: 10,000 RPM
        * disk transfer rate: 62-107 MB/sec
        * interface transfer rate: 320 MB/sec (Ultra320 SCSI)
        * sustained data rate: 40-70 MB/sec
        * data buffer (i.e., cache on controller): 8 MB


Bus

  address lines
  data lines
  control lines (read, write, interrupt request, ...)


Bus and disk interface standards

  PC bus: ISA, EISA, VESA, PCI, ...
  SPARC bus: Sbus, Mbus, Xbus, ...

  PC disk: SCSI, IDE, EIDE, ...
    SCSI slow, but handles 7 devices 
    IDE  slow, limited to 2 small disks 
    EIDE fast, handles disks, CDROMs, etc. 
    wide SCSI fastest
    EIDE drives cheaper than SCSI 


RAID storage - "Redundant Array of Inexpensive Disks"
  (now common to see "Redundant Array of Independent Disks")

  RAID    --concurrent--   redundancy   striping    number
  level   reads   writes     level                 of disks
  -----   -----   ------   ----------   --------   --------
    0      yes     yes       none         block      n
    1      yes     no      mirroring      none       2n
    2      no      no         ECC         bit        n+k
    3      no      no        parity     bit/byte     n+1
    4      yes     no        parity       block      n+1
    5      yes     yes    distributed     block      n+1
    6      yes     yes    distributed     block      n+2
  10/01    yes     no      mirroring      block      2n


  level 0 - no redundancy but parallel access, called "striping"
            * n disks

  level 1 - disk mirroring/shadowing
            * 2n disks
            * multiple, simultaneous I/Os are allowed
            * each write goes to two disks
            * read from either (e.g., choose one with shortest seek time
              or with fewest queued requests)

  level 2 - bit interleaved array - similar to ECC
            * n+k disks
            * dedicated ECC disks
            * expensive and uncommon 

  level 5 - rotated parity
            * n+1 disks
            * rotated parity allows multiple, simultaneous writes 
            * most popular among levels 3,4,5

  level 10 - striped mirrors - 2n disks with n-way striping of n disk pairs

  level 01 - mirrored stripes - 2n disks with mirroring of n-way striped disks


  commercial systems emphasize reliability and use high quality disks, also
    use redundant power supplies, etc., to remove single points of failure

  enhancements include battery-backed write-back cache in controller and
    dynamic hot sparing

  adaptive RAID allows segments to be dynamically reconfigured at level 1 or
    level 5 (background relocation of sectors, called "rebalancing")