HP ProLiant 4500 Disk Subsystem Performance and Scalability - Page 11

WHITE PAPER (cont.) Note: The actual transfer rates listed in Table 4 depend on the type of I/O being performed in the system. ECG025.0997 ... Disk Transfer Rates Hardware manufacturers calculate and define disk transfer rates as being the theoretical threshold for transferring data from the disk to the computer. For example, if you were to place one drive with an average transfer rate of 5 MB/s (see 1 in Figure 4) in a system, theoretically it would take four disks to saturate a SCSI channel with a transfer rate of 20 MB/s (see 2 in Figure 4). If you were to saturate the disk subsystem by adding drives, concurrency would increase because the system is able to process more I/O requests. Thus, increasing overall throughput, which improves system performance. A detailed discussion on concurrency is provided later in this document. It is important to note the difference between average disk sustained transfer rate and the transfer rate of the SCSI bus. In the disk transfer example above, the average disk sustained transfer rate refers to the disk transferring data at 5 MB/s. This transfer rate is a completely separate performance rating than the SCSI bus transfer rate, which in our example is 20 MB/s. Disk drives have a special interface used to communicate with the SCSI bus. This interface, defined in disk drive characteristic specification documents, identifies the type of controller the drive supports not the transfer rate of the disk. For example, if you are using a Wide-Ultra drive you know that this drive supports the Wide-Ultra SCSI Controller, which transfers at 40 MB/s but the average disk sustained transfer rate for the drive might be 5 MB/s. The earlier example listed above provides a simple illustration of a disk transferring data at 5 MB/s. However, some hard disks being manufactured today transfer data faster than the disk in the example. Table 4 lists the transfer rate specifications for all of the hard disk drives used during lab testing. Disk Capacity 2.1 GB 4.3 GB 9.1 GB Table 4: Hard Disk Transfer Rates Defined Transfer Rate Average Sustained Transfer Rate Up to 40 MB/s 4 MB/s Up to 40 MB/s 5 MB/s Up to 40 MB/s 7 MB/s 11