21
4.5
Performance characteristics
Drive performance is characterized by the following parameters:
Command overhead
Mechanical positioning
-
Seek time
-
Latency
Data transfer speed
Buffering operation (Look ahead/Write cache)
All the above parameters contribute to drive performance. There are other parameters that contribute to the
performance of the actual system. This specification defines the characteristics of the drive, not the characteristics
of 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 the time H->D Reg FIS w/ command bit is sent by host
to the time
PIO Set Up FIS is sent by device(PIO Read/Write)
DATA FIS is sent by device(DMA Read)
DMA Activate FIS is sent by device(DMA Write)
DMA Set Up FIS is sent by device(NCQ Read/Write)
Seek Start (Read cache not hit or Seek)
The table below gives average command overhead.
Command type
(Drive is in quiescent state)
Typical time (ms)
Typical time for NCQ
command (ms)
Read (Cache not hit)
0.5
0.5
Read (Cache hit)
0.1
0.2
Write
0.015
0.2
Seek
0.5
not applicable
Table 5 Command overhead
