HP ProLiant 4500 Disk Subsystem Performance and Scalability - Page 9

ECG025.0997 WHITE PAPER (cont.) ... Average Access Time Average access time is simply described as average seek time plus latency. What this equates to is the amount of time the disk has to seek to find the data plus the time it takes for the disk to spin under the head. For example, Figure 3 contains a disk with two tracks of data on it. Track 1 contains data sectors 1 - 8. Track 2 contains data sectors A - H. Thus, in our example, the disk head has to move (or seek) from the current position (track 1, sector 1) to the track you want to read (track 2, sector C). For the purpose of our illustration, Figure 3 displays the disk head performing these functions separately. However, in reality the disk drive performs both seek and latency functions simultaneously. Average Seek Time + Latency = Average Access Time 4 D 3C 5 E F Data 6 Latency 2B A 1 Seek Time G7 H 8 Unused space Disk Head Figure 3: Average Access Time Transfer Rates A disk subsystem is made up of multiple hardware components that communicate by transferring data to and from the disk(s) to a computer. The main parts of a disk subsystem are as follows: • Hard Disks • SCSI Channel • Disk Controller • I/O Bus • File System and Disk Controller Caching 9