Ultrastar 7K2
HGST Hard Disk Drive Technical Reference Manual
23
Perpendicular Magnetic Recording (PMR)
In perpendicular magnetic recording (PMR), the magnetization of each data bit is aligned vertically to the
spinning disk, rather than longitudinally as has been the case in hard drive technology for decades. In
longitudinal recording, as the bits become smaller and closer together, they experience an increasing
demagnetizing field, much like two bar magnets that are placed end-to-end repel one another. A property
of the media called coercivity must be increased to counteract the demagnetization to keep the bits stable
under thermal fluctuations; otherwise data corruption may occur over time. Higher media coercivity has
pushed the recording head write field to the limit of known materials.
In perpendicular recording, the adjacent bits attract instead of repel (as with bar magnets placed side by
side,) creating more thermally stable bits. In addition, the media contains a magnetically soft underlayer
(SUL) beneath the recording layer. This SUL allows a larger effective write field, thus higher coercivity
media, enabling further increases in density. Lastly, because of the vertical orientation of the bits, the
PMR recording layer tends to be thicker than that used for longitudinal recording, providing increased
signal for the read heads. All of these benefits enable HGST engineers to reliably pack more data on a given
disk than is possible with conventional longitudinal recording.
IntelliSeek
This unique IntelliSeek technology proactively calculates an optimum seek speed to eliminate hasty
movement of the actuator that produces noise and requires power, which is common in other drives. With
IntelliSeek, the actuator’s movement is controlled so the head reaches the next target sector just in time to
read the next piece of information, rather than rapidly accelerating and waiting for the drive rotation to
catch up. This smooth motion reduces power usage by more than 60 percent compared with standard
drives, as well as quiets seek operation and lowers vibration.
Native Command Queuing (NCQ)
These drives support Native Command Queuing. NCQ is a true Enterprise feature for environments such as
database, Web servers, and e-mail servers.
Performance of a random I/O workload can be improved through intelligent re-ordering of the I/O
requests so they read/write to and from the nearest available sectors and minimize the need for additional
disk revolutions or head actuator movement. This improvement is achieved though Native Command
Queuing (NCQ).
NCQ allows the drive to re-order read commands, thereby increasing random read IOPs. Additional NCQ
features that can prove beneficial include a Write Cache disabled IOP increase and a queuing
implementation built upon an existing, highly automated cache architecture. Queued reads in NCQ
leverage the same re-ordering schemes used for write caching. The firmware design maintains the "order"
of overlapping/colliding queued commands. NCQ is designed to excel in multi-threaded environments
with high random I/O loads.
Pre-emptive Wear Leveling (PWL)
This feature provides a solution for protecting the recording media against mechanical wear. In cases
where the drive is so busy with incoming commands that it is forced to stay in a same cylinder position
for a long time, the PWL control engine initiates forced seeks so that disk lubricant maintains an even
distribution and does not become depleted. This feature ensures reliability for applications that perform a
high incidence of read/write operations at the same physical location on the disk.
MicroFemto Slider
These drives incorporate the next generation of femto slider form factor in which the read/write head is
mounted on the small, lightweight microfemto slider that allows the head to move more quickly from track
to track on the disk.
HGST’s microfemto heads enhance tracking and increase shock tolerance,
producing a highly stable high-density drive platform.
