Transfer Rates - Compaq 108164-003 - ProLiant - 800 White Paper

ECG025.0997
W P
HITE APER
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Average Access Time
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Average access time is simply described as average seek time plus latency. What this
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equates to is the amount of time the disk has to seek to find the data plus the time it takes
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for the disk to spin under the head. For example, Figure 3 contains a disk with two tracks
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of data on it. Track 1 contains data sectors 1 – 8. Track 2 contains data sectors A – H.
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. . Thus, in our example, the disk head has to move (or seek) from the current position (track
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1, sector 1) to the track you want to read (track 2, sector C).
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For the purpose of our illustration, Figure 3 displays the disk head performing these
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functions separately. However, in reality the disk drive performs both seek and latency
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functions simultaneously.
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Average Seek Time + Latency = Average Access Time
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Latency
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Figure 3: Average Access Time
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Transfer Rates

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. . A disk subsystem is made up of multiple hardware components that communicate by
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transferring data to and from the disk(s) to a computer. The main parts of a disk
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. . subsystem are as follows:
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. . Hard Disks
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. . SCSI Channel
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. . Disk Controller
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. . I/O Bus
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. . File System and Disk Controller Caching
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9
5
4
E
D
3
C
B
2
A
H
8
1
Seek Time
Disk Head
Data
6
F
G
7
Unused space