Compaq 108164-003 - ProLiant - 800 White Paper page 17

ECG025.0997
W P
HITE APER
(cont.)
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Concurrency is very effective in a multi-drive environment because, while one drive is
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retrieving data, another request can be coming across the SCSI bus as shown in
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Figure 7. When using multiple drives, each drive can send data across the SCSI bus as
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soon as it is available. As more drives are added to the system, the busier the SCSI bus
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becomes. Eventually the SCSI bus becomes so busy that it yields no idle time.
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SCSI Bus
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Idle
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Drive 3
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Drive 2
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Drive 1
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First I/O Request
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Figure 7: Concurrency taking place in a multiple drive configuration.
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In conclusion, without concurrency (shown in Figure 6) the SCSI bus remains idle 60% of
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the time. In contrast, when using concurrency (shown in Figure 7) the SCSI bus remains
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busy 100% of the time and the subsystem is able to transfer more I/O requests in a
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shorter period of time.
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In the next few sections, we apply the knowledge learned earlier in this document to
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analyze the test results for Like Drive, Like Capacity and Disk Controller Scaling.
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L D S
I K E R I V E C AL A B I L I T Y
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Hardware scalability is difficult to accomplish and to maintain. The right balance or
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mixture is crucial for an effective disk subsystem. It is important to remember to balance
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the current performance needs with future disk capacity and performance requirements.
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For this reason, you need to choose the best performance configuration for the current
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disk subsystems, and at the same time allow enough room in the configuration to fulfill
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17
Legend
Request for Data (SCSI Bus)
Retrieving Data (Disk Drive)
Sending Data (SCSI Bus)
Idle Time (Disk Drive)
Idle Time (SCSI Bus)
3 3 3
2 2 2
1
1 4
Time
6 6 6
5 5 5
4
4