Magnetic Disks ■Read-write head 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 to 1000(on inner tracks)to 1000 to 2000(on outer tracks) ■To read/rite 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-7th Edition 12.7 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 12.7 ©Silberschatz, Korth and Sudarshan th Edition Magnetic Disks ▪ Read-write head ▪ 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 to 1000 (on inner tracks) to 1000 to 2000 (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. ▪ Cylinderi consists of i th track of all the platters