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Disk storage, basic file structures, hashing and modern storage…

- - - - operated on directly by the computer’s central processing unit (CPU),
        
        cache memory, which is a static RAM； used by the CPU to speed up execution of program instructions using techniques such as prefetching and pipelining.
        
        contents of main memory are lost in case of power failure or a system crash
        
        called main memory； DRAM (dynamic RAM), which provides the main work area for the CPU for keeping program instructions and data
        
        Programs reside and execute in dynamic random-access memory ( DRAM ).
    - - magnetic disk
        
        必须先进入主存，才能由CPU处理
        
        . Generally, large permanent databases reside on secondary storage (magnetic disks), and portions of the database are read into and written from buffers in main memory
      - flash memories （solid-state drive (SSD）
        
        nonvolatile
        
        fast access speed
        
        entire block must be erased and written over simultaneously
        
        used in cameras, MP3/MP4 players, cell phones, PDAs (personal digital assistants), and so on. USB
        
        two types
        
        NAND and NOR flash based on the type of logic circuits used， NAND flash devices have a higher storage capacity for a given cost
        
        used in cameras,
        MP3/MP4 players, cell phones, PDAs
    - - Optical disks (CD-ROMs, DVDs, and other similar storage media) and tapes
        
        offline
        storage
        
        larger capacity, cost less, and
        provide slower access to data
        
        CD-ROMs contain prerecorded data
        that cannot be overwritten
        
        magnetic tapes are used for archiving and backup storage of data. Tape jukeboxes
        
        NASA’s EOS (Earth Observation
        Satellite) system stores archived databases in this fashion.
  - - - A heap file (or unordered file)
      - a sorted file (or sequential
        file)
      - A hashed file
      - B-trees
    - - index
        
        ???
- - - - for random block reads this
        strategy is counterproductive.
    - - elevator algorithm; this algorithm mimics the behavior of an elevator that schedules requests on multiple floors
    - - Having the data file and the log file on
        the same disk is a cheaper solution but compromises performance.
  - - - apes serve a very important function—
        backing up the database
        
        airline reservation systems, to avoid any downtime, mirrored systems are used to keep three sets of identical disks—two in online operation and one as backup
- - - - controls the main memory
        directly, as in most RDBMSs
      - allocates buffers in virtual memory, which allows the control to transfer to the operating system (OS)
        
        common in main memory database systems and some object-oriented DBMSs
      - keeps two types of info about each page
        
        A pin-count: the number of times that page has been requested, or the number of current users of that page. If this count falls to zero, the page is considered unpinned.
        
        A dirty bit, which is initially set to zero for all pages but is set to 1 whenever that page is updated by any application program
  - - - throw out that page that has not been used (read or written) for the longest time
    - - Buffers with a 0 are vulnerable and may be used for
        replacement and their contents read back to disk.
        
        When a block is read into a buffer, the flag is set to 1. When the
        buffer is accessed, the flag is set to 1 also.
    - - when a buffer is required, the one that has been occupied the longest by a page is used for replacement.
        
        counter to
        desirable behavior
    - - when a block that is used most recently is not needed until all the remaining blocks in the relation are
- - - - commands/program variables
        
        file organization refers to the organization of the data of
        a file into records, blocks, and access structures
        
        access method provides a group of operations
        
        cannot apply an indexed access method to a file without an index