Performance Measures (Cont.) Disk block is a logical unit for storage allocation and retrieval 4 to 16 kilobytes typically Smaller blocks:more transfers from disk Larger blocks:more space wasted due to partially filled blocks Sequential access pattern Successive requests are for successive disk blocks Disk seek required only for first block Random access pattern Successive requests are for blocks that can be anywhere on disk Each access requires a seek Transfer rates are low since a lot of time is wasted in seeks l/O operations per second(IOPS) Number of random block reads that a disk can support per second 50 to 200 IOPS on current generation magnetic disks Database System Concepts-7th Edition 12.10 @Silberschatz,Korth and SudarshanDatabase System Concepts - 7 12.10 ©Silberschatz, Korth and Sudarshan th Edition Performance Measures (Cont.) ▪ Disk block is a logical unit for storage allocation and retrieval • 4 to 16 kilobytes typically ▪ Smaller blocks: more transfers from disk ▪ Larger blocks: more space wasted due to partially filled blocks ▪ Sequential access pattern • Successive requests are for successive disk blocks • Disk seek required only for first block ▪ Random access pattern • Successive requests are for blocks that can be anywhere on disk • Each access requires a seek • Transfer rates are low since a lot of time is wasted in seeks ▪ I/O operations per second (IOPS) • Number of random block reads that a disk can support per second • 50 to 200 IOPS on current generation magnetic disks