Compaq 108164-003 - ProLiant - 800 White Paper page 8

Note: Remember that the disks
used in Figure 2 are identical in
size and RAID configuration.
ECG025.0997
W P
HITE APER
(cont.)
. .
. .
However, be aware that the average latency time might not always decrease when adding
. .
. .
more drives to your system. For example, in Figure 2 - Example 2, the new configuration
. .
shows that the amount of time it takes to retrieve the data from sector B is actually longer
. .
. .
than the initial configuration. The reason for this is that the disk has to spin half way
. .
around to read sector B. In the initial configuration the disk only had to spin one-eighth a
. .
. .
revolution to read the identical data. But, keep in mind that the initial configuration for
. .
Example 2 required both seek time and latency time.
. .
. .
. .
. .
. .
. .
Initial
. .
Configuration
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
New
. .
. .
Configuration
. .
. .
. .
7
. .
. .
. . 5
. .
. .
. .
3
. .
. .
. .
1
. .
Example 1
. .
. .
. . Disk Head
. .
. .
Figure 2: Average Latency
. .
. .
Overall, these examples show us that in some configurations, as shown in our first
. .
. .
example, drive scaling would be a definite performance advantage. However, in other
. .
configurations it is not clear if you receive a performance gain because of the components
. .
. .
involved, such as the combination of average seek time and average latency time used in
. .
Figure 2 – Example 2.
. .
. .
. .
When you combine these terms (average seek time and average latency time), you define
. .
. .
another disk measurement called average access time, which is discussed in the
. .
upcoming section. From the information provided in this section, we know seek time plus
. .
. .
latency (or average access time) is a key in determining if performance is truly enhanced
. .
in your system.
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
. .
8
4
D
3
C
Example 1
B
2
A H
1
Disk Head
A
C
Example 2
E
G
5
E
6
F
G
7
Example 2
8
B
8
6
4
2 H
Disk Head
D
F