Head And Cylinder Skewing - Quantum FIREBALL PLUS AS 10.2 Product Manual

The requested read data takes up a certain amount of space in the cache segment.
Hence, the corresponding prefetch data can essentially occupy the rest of the space
within the segment. The other factors determining prefetch size are the maximum
and minimum prefetch. The drive's prefetch algorithm dynamically controls the
actual prefetch value based on the current demand, with the consideration of
overhead to subsequent commands.
5.3.2 5.3.2 5.3.2 5.3.2

Head and Cylinder Skewing

Head and Cylinder Skewing Head and Cylinder Skewing Head and Cylinder Skewing
Head and cylinder skewing in the Quantum Fireball Plus AS AT hard disk drives
minimize latency time and thus increases data throughput.
5.3.2.1 5.3.2.1 Head Skewing Head Skewing
5.3.2.1 5.3.2.1 Head Skewing Head Skewing
Head skewing reduces the latency time that results when the drive must switch
read/write heads to access sequential data. A head skew is employed such that the
next logical sector of data to be accessed will be under the read/write head once the
head switch is made, and the data is ready to be accessed. Thus, when sequential
data is on the same cylinder but on a different disk surface, a head switch is needed
but not a seek. Since the sequential head-switch time is well defined on the
Quantum Fireball Plus AS drives, the sector addresses can be optimally positioned
across track boundaries to minimize the latency time during a head switch. See
Table 5-2.
5.3.2.2 5.3.2.2 5.3.2.2 5.3.2.2 Cylinder Skewing Cylinder Skewing Cylinder Skewing Cylinder Skewing
Cylinder skewing is also used to minimize the latency time associated with a single-
cylinder seek. The next logical sector of data that crosses a cylinder boundary is
positioned on the drive such that after a single-cylinder seek is performed, and
when the drive is ready to continue accessing data, the sector to be accessed is
positioned directly under the read/write head. Therefore, the cylinder skew takes
place between the last sector of data on the last head of a cylinder, and the first
sector of data on the first head of the next cylinder. Since single-cylinder seeks are
well defined on the Quantum Fireball Plus AS drives, the sector addresses can be
optimally positioned across cylinder boundaries to minimize the latency time
associated with a single-cylinder seek. See Table 5-2.
5.3.2.3 5.3.2.3 Skewing with ID-less Skewing with ID-less
5.3.2.3 5.3.2.3 Skewing with ID-less Skewing with ID-less
In the ID-less environment, the drive's track and cylinder skewing will be based in
unit of wedges instead of the traditional sectors. The integrated µprocessor, disk
controller and ATA interface contains a "Wedge Skew Register" to assist in the task
of skewing, where the skew offset must now be calculated with every read/write
operation. The firmware will program the skew offset into this register every time
the drive goes to a new track. The integrated µprocessor, disk controller and ATA
interface will then add this value to the wedge number in the ID calculator,
effectively relocating the "first" sector of the track away from the index. For
example, if without skew, sector 0 is to be found following wedge 0, then if the skew
register is set to 10, sector 0 will be found following wedge 10.
Since the wedge-to-wedge time is constant over the entire disk, a single set of head
and cylinder skew off-sets will fulfill the requirement for all recording zones.
Quantum Fireball Plus AS 10.2/20.5/30.0/40.0/60.0 GB AT
Basic Principles of Operation
5-12