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FP-2.ppt FP-2.ppt Presentation Transcript

  • File Processing : Storage Media 2008, Spring Pusan National University Ki-Joune Li
  • Major Functions of Computer
    • Computation
    • Storage
    • Communication
    • Presentation
  • Storage of Data
    • Major Challenges
      • How to store and manage a large amount of data
        • Example : more than 100 peta bytes for EOS Project
      • How to represent sophisticated data
  • Modeling and Representation of Real World
    • Example
      • Building DB about Korean History
      • Very complicated and Depending on viewpoint
    • Database Course : 2008 Fall semester
    Real World Computer World
  • Managing Large Volume of Data
    • Large Volume of Data
      • Cost for Storage Media
        • Not very important and negligible
      • Processing Time
        • Comparison between main memory and disk access time
          • RAM : several nanoseconds (10 -9 sec)
          • Disk : several milliseconds (10 -3 sec)
        • Time is the most valuable resource
        • Example
          • Retrieving a piece of data from 100 peta bytes DB
  • Managing Large Volume of Data
    • Management of Data
      • Secure Management
        • From hacking
        • From any kinds of disasters
      • Consistency of Data
        • Example
          • Failure during a flight reservation transaction
          • Concurrent transaction
  • Goals of File Systems
    • To provide with
      • 1. efficient Data Structures for storing large and complex data
      • 2. Access Methods for rapid search
      • 3. Query Processing Methods
      • 4. Robust Management of Transactions
  • Memory Hierarchy
    • Large Data Volume
      • Not be stored in main memory
      • But in secondary memory
    • Memory Hierarchy
    Cache Memory 256 K bytes Main Memory 1G bytes Secondary Memory 100 G bytes Tertiary Memory 100 Tera bytes Faster Cheaper
  • Flash Memory
    • Non-Volatile
      • Data survives power failure, but
      • Data can be written at a location only once, but location can be erased and written to again
        • Can support only a limited number of write/erase cycles.
        • Erasing of memory has to be done to an entire bank of memory
    • Speed
      • Reads are roughly as fast as main memory
      • But writes are slow (few microseconds), erase is slower
    • Cost per unit of storage roughly similar to main memory
    • Widely used in embedded devices such as digital cameras
  • Optical Storage
    • Non-volatile :
      • data is read optically from a spinning disk using a laser
      • CD-ROM (800 MB), DVD (4.7 to 17 GB), CD-R, DVD-R
      • CD-RW, DVD-RW, and DVD-RAM
    • Speed
      • Reads and writes are slower than with magnetic disk
    • Juke-box systems
      • Large numbers of removable disks,
      • Few drives, and
      • Mechanism for automatic loading/unloading of disks
      • For storing large volumes of data
  • Tape
    • Non-volatile
      • Primarily Used for backup
    • Speed
      • Sequential access : much slower than disk
    • Cost
      • Very high capacity (40 to 300 GB tapes available)
      • Tape can be removed from drive
      • Drives are expensive
    • Tape jukeboxes
      • hundreds of terabytes to even a petabyte
  • Data Access with Secondary Memory Main Memory Access Request If in main memory Disk If not in main memory How to increase hit ratio ? Get Data Access to Disk Load on main memory Get Data Hit Ratio r h = n h / n a
  • Why Hit Ratio is so important ?
    • Example
      • for(int i=0;i<1000;i++)
        • Nbytes=read(fd,buf,100);
    1000 * 10 -2 sec = 10 sec 1000 * 10 -8 sec = 10 -5 sec 1000 disk accesses ? when r h = 0 when r h = 1
  • Physical Structure of Disk 512 bytes 200~400 sectors 2 * n DF
  • Disk Access Time
    • Disk Access Time
      • t = t S + t R + t T , where
      • t S : Seek Time
        • Time to reposition the head over the correct track
        • Average seek time is 1/2 the worst case seek time
        • 4 to 10 milliseconds on typical disks
      • t R : Rotational Latency
        • Time to reposition the head over the correct sector
        • Average rotational latency : ½ r (to find index point) + ½ r = r
        • In case of 15000 rpm : r =1*60sec/15000 = 4 msec
      • t T : Transfer Time
        • Time to transfer data from disk to main memory via channel
        • Proportional to the number of sectors to read
        • Real transfer time is negligible
  • Block-Oriented Disk Access
    • Example
      • for(int i=0;i<1000;i++)
        • Nbytes=read(fd,buf,10);
    1000 times 10 bytes Buffer in main memory 1024 bytes 10 times 100 times 1 block (e.g. 1024 bytes) Number of Disk Accesses
  • Disk Block
    • Unit of Disk Access
    • Block Size
      • Normally multiple of sectors
      • 1K, 4K, 16K or 64K bytes depending on configuration
    • Why not large block ?
      • Limited by the size of available main memory
      • Too large : unnecessary accesses of sectors
        • e.g. only 100 bytes, when block size is given as 64K
          • 1 block : 128 sectors (about ½ track, ½ rotation, 2 msec)
          • Too wasteful