Compaq 307560-001 Disk Subsystem Performance and Scalability - Page 16

Concurrency

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ECG025.0997 WHITE PAPER (cont.) ... Concurrency Concurrency is the process of eliminating the wait time involved to retrieve and return requested data. It takes place when multiple slow devices (e.g., disk drives) place I/O requests on a single faster device (e.g., SCSI bus). As shown in Figure 6, a request for data comes across the SCSI Bus asking the disk drive to retrieve some information. The disk drive retrieves then sends the requested data back to the server via the SCSI bus. The time it takes to complete this process seems to be acceptable at first glance until you examine the amount of time the SCSI bus remains idle. This idle time shown in Figure 6 is the amount of time the SCSI bus is waiting for the disk drive to complete the request. This valuable time could be used more efficiently in an environment taking advantage of concurrency. Legend Request for Data (SCSI Bus) Retrieving Data (Disk Drive) Sending Data (SCSI Bus) Idle Time (SCSI Bus) SCSI Bus Idle Drive 1 1 1 12 First I/O Request 2 23 Time 3 34 4 4 Figure 6: I/O request timing diagram for a single drive configuration. 16

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