Chapter 10: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 Database System Concepts-6th Edition 10.2 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.2 ©Silberschatz, Korth and Sudarshan th Edition Chapter 10: 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
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. Database System Concepts-6th Edition 10.3 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 10.3 ©Silberschatz, Korth and Sudarshan th Edition 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 batterbacked up main-memory
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. Database System Concepts-6th Edition 10.4 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.4 ©Silberschatz, Korth and Sudarshan th Edition 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
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(10K-1M)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 Widely used in embedded devices such as digital cameras, phones,and USB keys Database System Concepts-6th Edition 10.5 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.5 ©Silberschatz, Korth and Sudarshan th Edition 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 (10K – 1M) 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 Widely used in embedded devices such as digital cameras, phones, and USB keys
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 Sudarshan
Database 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
Physical Storage Media(Cont.) Optical storage non-volatile,data is read optically from a spinning disk using a laser CD-ROM(640 MB)and DVD(4.7 to 17 GB)most popular forms Blu-ray disks:27 GB to 54 GB Write-one,read-many (WORM)optical disks used for archival storage(CD-R,DVD-R,DVD+R) Multiple write versions also available(CD-RW,DVD-RW, DVD+RW,and DVD-RAM) Reads and writes are slower than with magnetic disk Juke-box systems,with large numbers of removable disks,a few drives,and a mechanism for automatic loading/unloading of disks available for storing large volumes of data Database System Concepts-6th Edition 10.7 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.7 ©Silberschatz, Korth and Sudarshan th Edition Physical Storage Media (Cont.) Optical storage non-volatile, data is read optically from a spinning disk using a laser CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms Blu-ray disks: 27 GB to 54 GB Write-one, read-many (WORM) optical disks used for archival storage (CD-R, DVD-R, DVD+R) Multiple write versions also available (CD-RW, DVD-RW, DVD+RW, and DVD-RAM) Reads and writes are slower than with magnetic disk Juke-box systems, with large numbers of removable disks, a few drives, and a mechanism for automatic loading/unloading of disks available for storing large volumes of data
Physical Storage Media(Cont.) Tape storage non-volatile,used primarily for backup(to recover from disk failure),and for archival data sequential-access-much slower than disk very high capacity(40 to 300 GB tapes available) tape can be removed from drive storage costs much cheaper than disk,but drives are expensive Tape jukeboxes available for storing massive amounts of data hundreds of terabytes(1 terabyte 109 bytes)to even multiple petabytes(1 petabyte 1012 bytes) Database System Concepts-6th Edition 10.8 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.8 ©Silberschatz, Korth and Sudarshan th Edition Physical Storage Media (Cont.) Tape storage non-volatile, used primarily for backup (to recover from disk failure), and for archival data sequential-access – much slower than disk very high capacity (40 to 300 GB tapes available) tape can be removed from drive storage costs much cheaper than disk, but drives are expensive Tape jukeboxes available for storing massive amounts of data hundreds of terabytes (1 terabyte = 109 bytes) to even multiple petabytes (1 petabyte = 1012 bytes)
Storage Hierarchy cache main memory flash memory magnetic disk optical disk magnetic tapes Database System Concepts-6th Edition 10.9 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 10.9 ©Silberschatz, Korth and Sudarshan th Edition Storage Hierarchy
Storage Hierarchy (Cont.) primary storage:Fastest media but volatile(cache,main memory). secondary storage:next level in hierarchy,non-volatile, moderately fast access time also called on-line storage E.g.flash memory,magnetic disks tertiary storage:lowest level in hierarchy,non-volatile,slow access time also called off-line storage E.g.magnetic tape,optical storage Database System Concepts-6th Edition 10.10 @Silberschatz,Korth and Sudarshan
Database System Concepts - 6 10.10 ©Silberschatz, Korth and Sudarshan th Edition Storage Hierarchy (Cont.) primary storage: Fastest media but volatile (cache, main memory). secondary storage: next level in hierarchy, non-volatile, moderately fast access time also called on-line storage E.g. flash memory, magnetic disks tertiary storage: lowest level in hierarchy, non-volatile, slow access time also called off-line storage E.g. magnetic tape, optical storage
Magnetic Hard Disk Mechanism track t spindle arm assembly sector s、 cylinder c-→l read-write head platter arm rotation NOTE:Diagram is schematic,and simplifies the structure of actual disk drives Database System Concepts-6th Edition 10.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 6 10.11 ©Silberschatz, Korth and Sudarshan th Edition Magnetic Hard Disk Mechanism NOTE: Diagram is schematic, and simplifies the structure of actual disk drives