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. Average latency is 1/2 of the worst case latency. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) Data-transfer rate-the rate at which data can be retrieved from or stored to the disk. 25 to 100 MB per second max rate,lower for inner tracks Multiple disks may share a controller,so rate that controller can handle is also important E.g.ATA-5:66 MB/sec,SATA:150 MB/sec,Ultra 320 SCSI:320 MB/s Fiber Channel (FC2Gb):256 MB/s Database System Concepts-5th Edition,Oct 23,2005. 11.15 @Silberschatz,Korth and SudarshanDatabase System Concepts - 5 11.15 ©Silberschatz, Korth and Sudarshan th Edition, Oct 23, 2005. 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.  Average latency is 1/2 of the worst case latency.  4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) Data-transfer rate – the rate at which data can be retrieved from or stored to the disk. 25 to 100 MB per second max rate, lower for inner tracks Multiple disks may share a controller, so rate that controller can handle is also important  E.g. ATA-5: 66 MB/sec, SATA: 150 MB/sec, Ultra 320 SCSI: 320 MB/s  Fiber Channel (FC2Gb): 256 MB/s