Compaq 108164-003 - ProLiant - 800 White Paper page 16

ECG025.0997
W P
HITE APER
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Concurrency
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Concurrency is the process of eliminating the wait time involved to retrieve and return
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requested data. It takes place when multiple slow devices (e.g., disk drives) place I/O
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requests on a single faster device (e.g., SCSI bus). As shown in Figure 6, a request for
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data comes across the SCSI Bus asking the disk drive to retrieve some information. The
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. . disk drive retrieves then sends the requested data back to the server via the SCSI bus.
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The time it takes to complete this process seems to be acceptable at first glance until you
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examine the amount of time the SCSI bus remains idle. This idle time shown in Figure 6
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is the amount of time the SCSI bus is waiting for the disk drive to complete the request.
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This valuable time could be used more efficiently in an environment taking advantage of
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concurrency.
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SCSI Bus
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Idle
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Drive 1
1
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First I/O Request
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Figure 6: I/O request timing diagram for a single drive configuration.
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Legend
Request for Data (SCSI Bus)
Retrieving Data (Disk Drive)
Sending Data (SCSI Bus)
Idle Time (SCSI Bus)
1 1 2 2 2 2
Time
3
3 3 3 4 4
4
4