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 Capacities range up to roughly 1.5 TB as of 2009 Much larger capacity and cost/byte than main memory/flash memory Growing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years) Survives power failures and system crashes disk failure can destroy data,but is rare Database System Concepts-6th Edition 10.6 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 10.6 ©Silberschatz, Korth and Sudarshan th Edition 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 Capacities range up to roughly 1.5 TB as of 2009  Much larger capacity and cost/byte than main memory/flash memory  Growing constantly and rapidly with technology improvements (factor of 2 to 3 every 2 years) Survives power failures and system crashes  disk failure can destroy data, but is rare