Head And Cylinder Skewing - Maxtor D740X-6L 20.0 AT 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.
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Head and cylinder skewing in the Maxtor D740X-6L hard disk drives minimize
latency time and thus increases data throughput.
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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 Maxtor D740X-6L
drives, the sector addresses can be optimally positioned across track boundaries to
minimize the latency time during a head switch. See Table 5-2.
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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
Maxtor D740X-6L 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.
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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.
Maxtor D740X-6L 20.0/40.0/60.0/80.0GB AT
Basic Principles of Operation
5-12