HP ProLiant 4500 Disk Subsystem Performance and Scalability - Page 5

ECG025.0997 WHITE PAPER (cont.) ... Table 2: (cont.) Disk Performance Measurement Terms Terms Description Latency The time required for the disk to spin one complete revolution. Average Latency The time required for the disk to spin half a revolution. Average Access Time The average length of time it takes the disk to seek to the required track plus the amount of time it takes for the disk to spin the data under the head. Average Access Time equals Average Seek Time plus Latency. Transfer Rate The speed at which the bits are being transferred through an interface from the disk to the computer. Concurrency The number of I/O requests that can be processed simultaneously. RPM (Revolutions Per Minute) The measurement of the rotational speed of a disk drive on a per minute basis. Table 2 lists the definitions of disk-related performance characteristics. Let's now use those definitions in the next several sections to address how adding drives to your system can affect performance. Seek Time and Average Seek Time Seek time describes the time it takes for the disk head to move across the disk to find data on another track. The track of data you want could be adjacent to your current track or it could be the last track on the disk. Average seek time, however, is the average amount of time it would take the disk head to move to the track that holds the data. Generally, this average length of time will be the same amount of time it takes to seek half way across the disk and is usually given in milliseconds. 5