Magnetic Disks Read-write head Positioned very close to the platter surface(almost touching it) Reads or writes magnetically encoded information. Surface of platter divided into circular tracks Over 50K-100K tracks per platter on typical hard disks Each track is divided into sectors. A sector is the smallest unit of data that can be read or written. Sector size typically 512 bytes Typical sectors per track:500(on inner tracks)to 1000(on outer tracks) To read/write a sector disk arm swings to position head on right track platter spins continually;data is read/written as sector passes under head Head-disk assemblies multiple disk platters on a single spindle (1 to 5 usually) one head per platter,mounted on a common arm. Cylinder i consists of ith track of all the platters Database System Concepts-5th Edition,Oct 23,2005. 11.12 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 11.12 ©Silberschatz, Korth and Sudarshan th Edition, Oct 23, 2005. Magnetic Disks Read-write head Positioned very close to the platter surface (almost touching it) Reads or writes magnetically encoded information. Surface of platter divided into circular tracks Over 50K-100K tracks per platter on typical hard disks Each track is divided into sectors. A sector is the smallest unit of data that can be read or written. Sector size typically 512 bytes Typical sectors per track: 500 (on inner tracks) to 1000 (on outer tracks) To read/write a sector disk arm swings to position head on right track platter spins continually; data is read/written as sector passes under head Head-disk assemblies multiple disk platters on a single spindle (1 to 5 usually) one head per platter, mounted on a common arm. Cylinder i consists of i th track of all the platters