Performance Characteristics; Command Overhead; Mechanical Positioning; Figure 4. Command Overhead - IBM IC35L040AVVN07-0 - 40GB IDE Deskstar 120GXP 7200RPM UATA-100 07N9682 Specifications

4.4 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.
4.4.1 Command overhead
Command overhead is defined as the time required
! from the time the command is written into the command register 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.
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)

Figure 4. Command overhead

4.4.2 Mechanical positioning
4.4.2.1 Average seek time (without command overhead, including settling)
Command Type
Read (60 GB – 120 GB models)
Read (20 GB & 40 GB models)
Write
Read (Quiet Seek mode)
Write (Quiet Seek mode)

Figure 5. Mechanical positioning performance

Typical (ms)
8.2
8.5
9.2
19.5
20.5
Deskstar 120GXP hard disk drive specifications
13
Time (Typical)
(ms)
0.3
0.1
0.015
0.3
Max (ms)
9.2
9.5
10.2
20.5
21.5
Time (Typical) for
queued command
(ms)
0.3
0.1
0.05
not applicable