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4.5 Performance characteristics
The performance of a drive is characterized by the following parameters:
!
Command overhead
!
Mechanical head positioning
- Seek time
- Latency
!
Data transfer speed
!
Buffering operation (read ahead/write cache)
NOTE: All the above parameters contribute to drive performance. There are other parameters that contri-
bute to the performance of the actual system. This specification tries to define the bare drive character-
istics, not the system throughput, which depends on the system and the application.
4.5.1 Command overhead
Command overhead is defined as the time required:
!
from last byte of command phase
!
to the first byte of data phase
!
excluding
- Physical seek time
- Latency time
- Initiator delay with reconnections
<30
µ
s
Cache Hit
<400
µ
s
Cache Not Hit
Time
Read Command Case (Drive is in quiescent state)
Figure 6. Command overhead
Ultrastar 146Z10 hard disk drive specifications
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