Compaq 108164-003 - ProLiant - 800 White Paper page 11

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