Compaq 108164-003 - ProLiant - 800 White Paper page 10

Remember that the slowest disk
subsystem component determines
the overall throughput of the
system.
ECG025.0997
W P
HITE APER
(cont.)
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In order to share information, all of the disk subsystem components have to communicate
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with each other, as shown in Figure 4. The disk subsystem components communicate
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with each other using hardware interfaces such as Small Computer System Interface
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(SCSI) channels and Peripheral Component Interconnect (PCI) buses. These
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communication highways, called channels and/or buses, communicate at different rates of
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speed known as transfer rates.
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Each of the disk subsystem components transfer data at different rates. It is important to
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understand the different transfer rates of each component because this information helps
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you identify potential performance bottlenecks within your system. For example, Figure 4
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shows hard disks transferring data to the SCSI channel (bus), which transfers the
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information to the disk controller, which then passes the data to the Host Bus and then on
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to the server. If one hard disk transfers at 5 MB/s, the SCSI channel transfers at 40 MB/s,
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the disk controller transfers at 40 MB/s and the Host Bus transfers at 540 MB/s, it is
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obvious that the hard disk is the bottleneck. Therefore, by knowing the transfer rate of
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each subsystem device, potential bottlenecks can be easily identified and corrected.
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The key to improving system performance is focusing on how to maximize data
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throughput by minimizing the amount of time the disk subsystem has to wait to receive or
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send data. In the upcoming sections, we discuss how to identify performance bottlenecks
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and where they could possibly occur in your disk subsystem.
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More Drives
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1
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Controller transfers to/from disks via SCSI channel
2
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1 Disk drive average
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sustained transfer
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rate
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SCSI Bus
2
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transfer rate
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PCI Bus
3
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transfer rate
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Host Bus (Memory)
4
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transfer rate
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Figure 4: Disk subsystem components transferring data.
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10
PCI Bus transfers
3
to/from the Host Bus
Dual Inline Memory
Module (DIMM)
4
Host Bus transfers to/from
ProLiant 5000
Controller transfers
to/from PCI Bus