ch11-2 - Chapter 11 Storage and File Structure Overview of...

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1 ©Silberschatz, Korth and Sudarshan 11.1 Database System Concepts Chapter 11: Storage and File Structure Chapter 11: Storage and File Structure ! Overview of Physical Storage Media ! Magnetic Disks ! RAID ! Tertiary Storage ! Storage Access ! File Organization ! Organization of Records in Files ! Data-Dictionary Storage ! Storage Structures for Object-Oriented Databases ©Silberschatz, Korth and Sudarshan 11.2 Database System Concepts Classification of Physical Storage Media Classification of Physical Storage Media ! Speed with which data can be accessed ! Cost per unit of data ! Reliability " data loss on power failure or system crash " physical failure of the storage device ! Can differentiate storage into: " volatile storage : loses contents when power is switched off " non-volatile storage : # Contents persist even when power is switched off. # Includes secondary and tertiary storage, as well as batter- backed up main-memory.
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2 ©Silberschatz, Korth and Sudarshan 11.3 Database System Concepts Physical Storage Media Physical Storage Media ! Cache – fastest and most costly form of storage; volatile; managed by the computer system hardware. ! Main memory : " fast access (10s to 100s of nanoseconds; 1 nanosecond = 10 –9 seconds) " generally too small (or too expensive) to store the entire database # capacities of up to a few Gigabytes widely used currently # Capacities have gone up and per-byte costs have decreased steadily and rapidly (roughly factor of 2 every 2 to 3 years) " Volatile — contents of main memory are usually lost if a power failure or system crash occurs. ©Silberschatz, Korth and Sudarshan 11.4 Database System Concepts Physical Storage Media (Cont.) Physical Storage Media (Cont.) ! Flash memory " Data survives power failure " 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 " 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 " also known as EEPROM (Electrically Erasable Programmable Read-Only Memory)
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3 ©Silberschatz, Korth and Sudarshan 11.5 Database System Concepts Physical Storage Media (Cont.) Physical Storage Media (Cont.) ! Magnetic-disk " Data is stored on spinning disk, and read/written magnetically " Primary medium for the long-term storage of data; typically stores entire database. " Data must be moved from disk to main memory for access, and written back for storage # Much slower access than main memory (more on this later) " direct-access – possible to read data on disk in any order, unlike magnetic tape " Hard disks vs floppy disks " Capacities range up to roughly 100 GB currently # Much larger capacity and cost/byte than main memory/flash memory # Growing constantly and rapidly with technology improvements
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ch11-2 - Chapter 11 Storage and File Structure Overview of...

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