heads from touching the media itself. Lubricants on the media can build up on the head under certain circumstances and cause the head to fly too high. Lube buildup can also mean that uncorrupted, well-written data cannot be read because the read element is too far from the media. Lube buildup can be caused by the mechanical properties of the lubricant, which is dependent upon the chemical composition. Persistent high fly height can also be caused by specific operations. For example, when not writing or reading, if the head is left to sit above the same track while the disks spin, lubricant can collect on the heads. In some cases simply powering down the HDD will cause the heads to touch down (as they are designed to do) in the landing zone to disturb the lube buildup. This is very design specific, however, and does not always work.

During the manufacturing process, the surface of the HDD is checked and defects are mapped out, and the HDD firmware knows not to write in these locations. They also add “padding” around the defective area mapping out more blocks than the estimated minimum, creating additional physical distance around the defect that is not available for storing data. Since it is difficult to determine the exact length, width, and shape of a defect, the added padding provides an extra safeguard against writing on a media defect.

Media imperfections such as voids (pits), scratches, hydrocarbon contamination (various oils), and smeared soft particles can not only cause errors during writing, but also corrupt data after it has been written. The sputtering process used to apply some of the media layers can leave contaminants buried within the media. Subsequent contact by the slider can remove these bumps, leaving voids in which the media is defective. If data is already written there, the data is corrupted. If none is written, the next write process will be unsuccessful, but the user won’t know this unless a write-verify command is used.

Early reliability analyses assumed that once written, data will remain undestroyed except by degradation of the magnetic properties of the media, a process known as bit-rot. Bit-rot, in

which the magnetic media is not capable of holding the proper magnetic field to be correctly interpreted as a 0 or a 1, is really not an issue. Media can

Based on
degrade, but the probability of this
mode is inconsequential compared
technology and
with other modes. Data can become
design, where
corrupted whenever the disks are spin-
ning, even when data is not being writ-
must the developers
ten to or read from the disk. Common
causes for erasure include thermal
and architects
asperities, corrosion, and scratches or
place the efforts
smears.

Thermal asperities are instances of

for resiliency?
high heat for a short duration caused
by head-disk contact. This is usu-
ally the result of heads hitting small
“bumps” created by particles that re-
main embedded in the media surface
even after burnishing and polishing.
The heat generated on a single contact
can be high enough to erase data. Even
if not on the first contact, cumulative
effects of numerous contacts may be
sufficient to thermally erase data or
mechanically destroy the media coat-
ings and erase data.
The sliders are designed to push
away airborne particles so they do not
become trapped between the head
and disk surface. Unfortunately, re-
moving all particles that are in the 0.3
µ-in. range is very difficult, so particles
do get caught. Hard particles used in
the manufacture of an HDD, such as
Al O , Ti W, and C, will cause surface
23
scratches and data erasure. These
scratches are then media defects that
are not mapped out, so the next time
data is written to those locations the
data will be corrupted immediately.
Other “soft” materials such as stain-
less steel can come from assembly
tooling and aluminum from residuals
from machining the case. Soft parti-
cles tend to smear across the surface
of the media rendering the data un-
readable and unwritable. Corrosion,
although carefully controlled, can
also cause data erasure and may be ac-
celerated by high ambient heat within
the HDD enclosure and the very high
heat flux from thermal asperities.

 

Latent Defects Data Latent defects are the most insidious kinds of errors. These data corruptions are present on the HDD but undiscovered until the data is read. If no operational failures occur at the first