[CMPT 454] Week 1_1

June·2021년 1월 13일

CMPT 454

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DBMS Stores Data on Hard Disks

  • Why not store everything id main memoery?
    - Costs too much
    • Main memory is volatile to power off.
    • Typical storage hierarchy:
      • Main memory for currently used data.
      • Disk for main database
      • Tapses for archiving older versions
  • We consider main memory and disk.

Disks and Files

  • DBMS stores information on hard disks
  • This has major implications for DBMS design!
    • READ: transfer data from disk to main memory
    • WRITE: transfer data from main memory to disk
    • Called I/O operations: Both are high-cost operations due to mechanical components, relative to CPU operations, so must be planned carefully.


  • Main advantge over tapes: random access vs. sequential
  • Data is stored and retrieved in disk pages of fixed size, the smallest unit for data retrieval.
  • Time to retrieve a disk page depends upon location on disk.
    - Relative placement on disk has a major impact on performance!

Components of a Disk

block size == page size

Accessing a Disk page

  • Time to access (read/write) a disk page:
    • seek time (moving arms to position disk head on track)
    • rotational delay (waiting for page to rotate under head)
    • transfer time (actually moving data to/from disk surface)
  • Seek time and rotational delay dominate
    • Seek time varies from about 1 to 20msec
    • Rotational delay varies from 0 to 10msec
    • Transfer rate is about 1 msec per 4KB page
  • Reduce seek/rotational delays through careful arrangement of data pages on disk

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