Compaq 108164-003 - ProLiant - 800 White Paper page 14

ECG025.0997
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HITE APER
(cont.)
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For instance, let's say you request data stored on your disk drive (refer to Figure 4 for
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reference). The system first tries to complete the READ request by retrieving the data
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from the file system cache (memory). If it is not there, the system has to retrieve the data
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from the hard disk. Figure 5 shows the communications that take place to retrieve data
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from the disk.
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Operating system
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checks to see if the
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requested data is in
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memory.
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No
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Operating system sends the
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request through the I/O bus
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(i.e., PCI bus) to the disk controller.
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Disk controller
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checks its cache (i.e.,
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Array Accelerator) for
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the data.
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No
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Disk controller sends the request
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across the SCSI bus to the
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physical disk drives.
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Figure 5: Retrieving data from the hard disk.
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Now let's examine the same scenario if the requested information were located in file
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system cache (refer to Figure 5 if necessary). The operating system checks to see if the
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requested data is in memory (i.e., File system cache). Operating system passes the
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requested information to the application.
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As you can conclude from the flowchart example, the more information stored in memory
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the faster the system can access the requested data. Thus, if you are retrieving data from
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memory, the speed of the Host Bus will influence system performance.
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14
Operating system receives the data, passes it to
Yes
the requesting application and keeps a copy of
Disk controller keeps a copy of
the data in its cache and passes
the data through the I/O bus
Yes
Drive transfers the data across the
SCSI bus to the disk controller.
Drive head(s) seek to the track
where the data is located and reads
the request in its cache.
(i.e., PCI bus).
the data.