【正文】 ysical Storage Media (Cont.) ? Tape storage 磁帶 ? nonvolatile, used primarily for backup (to recover from disk failure), and for archival data 非易逝 ? sequentialaccess – much slower than disk ? 40 to 300 GB tapes available 大容量 ? cheaper than disk, but drives are expensive ? Tape jukeboxes available for storing massive amounts of data 海量磁帶庫 ? hundreds of terabytes (1 terabyte = 109 bytes) to even a petabyte (1 petabyte = 1012 bytes) 169。Silberschatz, Korth and Sudarshan Database System Concepts 5th Edition, Oct 23, 2022. Physical Storage Media ? Cache – fastest and most costly form of storage。Silberschatz, Korth and Sudarshan Database System Concepts 5th Edition, Oct 23, 2022. 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 易逝，揮發(fā) RAM ? nonvolatile storage: ? Contents persist even when power is switched off. ? Includes secondary and tertiary storage, as well as batterbacked up mainmemory. 磁盤，光盤，磁帶，閃存 169。Silberschatz, Korth and Sudarshan Database System Concepts 5th Edition, Oct 23, 2022. Physical Storage Media (Cont.) ? Optical storage 光存儲 ? nonvolatile, data is read optically from a spinning disk using a laser 非易逝 ? CDROM (640 MB) and DVD ( to 17 GB) most popular forms ? Writeone, readmany (WORM) optical disks used for archival storage (CDR and DVDR) ? Multiple write versions also available (CDRW, DVDRW, and DVDRAM) ? slower than with magic disk ? Jukebox systems, 多個可移動盤 169。Silberschatz, Korth and Sudarshan Database System Concepts 5th Edition, Oct 23, 2022. BufferReplacement Policies (Cont.) ? Pinned block（被釘住的塊） – memory block that is not allowed to be written back to disk. ? Most recently used (MRU) strategy – system must pin the block currently being processed. After the final tuple of that block has been processed, the block is unpinned, and it bees the most recently used block. ? Buffer manager can use statistical information regarding the probability that a request will reference a particular relation ? Buffer managers also support forced output （強制寫出） of blocks for the purpose of recovery 169。Silberschatz, Korth and Sudarshan Database System Concepts 5th Edition, Oct 23, 2022. VariableLength Records (Cont.) ? 定長 的實現(xiàn) Fixedlength representation: ? reserved space 保留空間 ? Pointers 指針 ? 保留空間 Reserved space – 暫時不用的 空著 16