Hard disks work by having the read/write heads fly over the surface of the disk
platters. However, this floating action occurs only when the platters are spinning. When
the platters are not moving, the air cushion dissipates, and the heads float down to
contact the surfaces of the platters. This contact occurs when the drive spins down, as
soon as the platters stop spinning fast enough, and occurs again when the spindle motor is
restarted, until the platters get up to speed. Each time the heads contact the surface of
the platters, there is the potential for damage. In addition to friction (and thus heat)
on these sensitive components, dust can be created as the heads scrape off minute amounts
of material from the platters.
Knowing that this will occur, manufacturers plan for it by incorporating special
lubricants, and by ensuring that the heads normally contact the platters in a special landing zone away from user data areas. While
obviously good planning, the entire way that starts and stops are handled--allowing
contact with the platters--is not a great design. After several decades of handling this
the same way, IBM engineers came up with a better solution.
Instead of letting the heads fall down to the surface of the disk when the disk's motor
is stopped, the heads are lifted completely off the surface of the disk while the drive is
still spinning, using special ramps. Only then are the disks allowed to spin down. When
the power is reapplied to the spindle motor, the process is reversed: the disks spin up,
and once they are going fast enough to let the heads fly without contacting the disk
surface, the heads are moved off the "ramps" and back onto the surface of the
platters. IBM calls this load/unload technology. In theory it should improve the
reliability of the hard disk as a whole. Unfortunately, I am unaware of any other drive
manufacturers using it at this time.
|
Diagram showing how IBM's load/unload
ramp technology functions. (One head and
surface is shown; there would be a different ramp
for each head and surface the disk has.) |
Original image © IBM Corporation
Image used with permission. |
This feature appeared at about the same time that IBM went to the use of glass substrate platters. It may well be that the
glass platters are more susceptible to damage from head contact than traditional aluminum
platters. Regardless of the reason, load/unload technology is a great idea that I hope is
adopted by other drive makers. This feature is also likely part of the reason why many IBM
drives have very high start/stop cycle specifications. The
only possible drawback I can see to this feature, other than possibly a bit of extra cost,
is that spin-up time might be a bit slower on
these drives.
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