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.SATA:150 MB/sec,SATA-lI 3Gb(300 MB/sec) Ultra 320 SCSI:320 MB/s,SAS (3 to 6 Gb/sec) Fiber Channel (FC2Gb or 4Gb):256 to 512 MB/s Database System Concepts-6th Edition 10.16 @Silberschatz,Korth and SudarshanDatabase System Concepts - 6 10.16 ©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.  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. SATA: 150 MB/sec, SATA-II 3Gb (300 MB/sec)  Ultra 320 SCSI: 320 MB/s, SAS (3 to 6 Gb/sec)  Fiber Channel (FC2Gb or 4Gb): 256 to 512 MB/s