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Seek Time, Latency, Data Transfer Time, The head, is here, Direction of, actuator, positioning, Seek - vs 15k 6

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Appendices: The Fundamentals of Disc Drive Performance APPENDIX A There is a single actuator (shown in Figure A1) in each drive. This actuator carries multiple heads in order to provide a separate head to read and write data on each side of each disc used in the drive. However, only one head can be active at a time due to limitations in track consistency from disc to disc. APPENDIX B A drive's access time is an essential factor in determining overall drive performance. Access time is defined as the time needed for the read/write Figure A1. head to reach the disc location where the desired data resides. In current storage configurations, this access time is much greater than the time required by the device's electronics to actually read or write the data (referred to as data transfer time). The two major components of a drive's access time are seek time and latency (Figure A2). 1. The head is here 2. Direction of actuator positioning: Seek Time 3. Direction of disc rotation: Latency 4. The data needed is here Figure A2. Seek Time: When the drive receives a request, the head moves, or seeks, across the disc to the track containing the desired data. The time it takes for the head to move from its initial position to the data's track is called seek time. Because 15K drives utilize smaller-diameter discs than 10K drives, the distance the head must travel is reduced, resulting in faster seek times. Latency: After the head is positioned on the appropriate track of the disc, the head must wait on that track until the rotating disc brings the desired sector of data underneath the head. The time it takes for the data to reach the head is called latency time. Of course, the faster the disc is spinning, the shorter the latency time. The total time required for the disc to make one complete revolution is 4.0 msec for a 15K drive (vs. 6.0 msec for a 10K drive). When a seeking head reaches the target track, the data may already be immediately under the head, resulting in an effective latency of 0 msec. In other cases, where the data has just passed beneath the head, the head must wait a full revolution (4.0 msec) before reading the data. Thus latency on a 15K-RPM drive averages out to 2.0 msec. Data Transfer Time: For the majority of performance applications, the time required for the electronics to read the data and transfer it to the bus (referred to as the data transfer rate) is negligible compared to the data access time; as such, the data transfer rate has relatively little impact on the drive's performance. 6

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Appendices: The Fundamentals of Disc Drive Performance
APPENDIX A
There is a single actuator (shown in Figure A1) in
each drive. This actuator carries multiple heads in
order to provide a separate head to read and write
data on each side of each disc used in the drive.
However, only one head can be active at a time
due to limitations in track consistency from disc to
disc.
APPENDIX B
A drive’s access time is an essential factor in
determining overall drive performance. Access
time is defined as the time needed for the read/write
head to reach the disc location where the desired
data resides. In current storage configurations, this access time is much greater than the time required by the device’s electronics to
actually read or write the data (referred to as data transfer time). The two major components of a drive’s access time are seek time
and latency (Figure A2).
Seek Time:
When the drive receives a request, the head moves, or seeks, across the disc to the track containing the desired data.
The time it takes for the head to move from its initial position to the data’s track is called seek time. Because 15K drives utilize
smaller-diameter discs than 10K drives, the distance the head must travel is reduced, resulting in faster seek times.
Latency:
After the head is positioned on the appropriate track of the disc, the head must wait on that track until the rotating disc
brings the desired sector of data underneath the head. The time it takes for the data to reach the head is called latency time. Of
course, the faster the disc is spinning, the shorter the latency time. The total time required for the disc to make one complete
revolution is 4.0 msec for a 15K drive (vs. 6.0 msec for a 10K drive).
When a seeking head reaches the target track, the data may already be immediately under the head, resulting in an effective latency
of 0 msec. In other cases, where the data has just passed beneath the head, the head must wait a full revolution (4.0 msec) before
reading the data. Thus latency on a 15K-RPM drive averages out to 2.0 msec.
Data Transfer Time:
For the majority of performance applications, the time required for the electronics to read the data and transfer
it to the bus (referred to as the data transfer rate) is negligible compared to the data access time; as such, the data transfer rate has
relatively little impact on the drive’s performance.
Figure A1.
Figure A2.
1.
The head
is here
2.
Direction of
actuator
positioning:
Seek Time
3.
Direction of
disc rotation:
Latency
4.
The data
needed
is here