Performance Measures of Disks Access time-the time it takes from when a read or write request is issued to when data transfer begins.Consists of: Seek time-time it takes to reposition the arm over the correcttrack. Average seek time is 1/2 the worst case seek time. Would be 1/3 if all tracks had the same number of sectors,and we ignore the time to start and stop arm movement 4 to 10 milliseconds on typical disks Rotational latency-time it takes for the sector to be accessed to appear under the head. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) Average latency is 1/2 of the above latency. Overall latency is 5 to 20 msec depending on disk model ■ Data-transfer rate-the rate at which data can be retrieved from or stored to the disk. 25 to 200 MB per second max rate,lower for inner tracks Database System Concepts-7th Edition 12.9 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 12.9 ©Silberschatz, Korth and Sudarshan th Edition Performance Measures of Disks ▪ Access time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of: • Seek time – time it takes to reposition the arm over the correct track. ▪ Average seek time is 1/2 the worst case seek time. • Would be 1/3 if all tracks had the same number of sectors, and we ignore the time to start and stop arm movement ▪ 4 to 10 milliseconds on typical disks • Rotational latency – time it takes for the sector to be accessed to appear under the head. ▪ 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) ▪ Average latency is 1/2 of the above latency. • Overall latency is 5 to 20 msec depending on disk model ▪ Data-transfer rate – the rate at which data can be retrieved from or stored to the disk. • 25 to 200 MB per second max rate, lower for inner tracks