Performance Characteristics; Mechanical Positioning; Table 4: Command Overhead; Table 5: Mechanical Positioning Performance - Hitachi HDT725025VLA380 - Deskstar 0A33423 T7K500 250GB SATA 7200RPM 8MB Cache Hard Drive Specifications

4.5 Performance characteristics

Drive performance is characterized by the following parameters:
• Command overhead
• Mechanical head 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 perfor-
mance of the actual system. This specification tries to define the bare drive characteristics, not 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 the command is written into the command regis-
ter by a host to the assertion of DRQ for the first data byte of a READ command when the requested data is not in
the buffer excluding Physical seek time and Latency.
The table below gives average command overhead.

Table 4: Command overhead

Command type (Drive is in quiescent state)
Read (cache not hit) (from Command Write to Seek Start)
Read (cache hit) (from Command Write to DRQ)
Write (from Command Write to DRQ)
Seek (from Command Write to Seek Start)

4.5.2 Mechanical positioning

4.5.2.1 Average seek time (without command overhead, including settling)

Table 5: Mechanical positioning performance

Command type
Read
Write
Read (Quiet Seek mode)
Write (Quiet Seek mode)
Typical (ms) Max (ms)
8.5
9.5 10.5
14.0 14.7
15.0 15.7
Deskstar T7K500 Hard Disk Drive Specification
15
Time (typical) Time (typical) for
(ms) queued command
(ms)
0.5
0.1
0.015
0.5
9.5
0.5
0.2
0.2
not applicable