Roy Sigurd Karlsbakk wrote:
Nope. Most HDDs today have a single read channel, and they select
which head uses that channel at any point in time. They cannot use
multiple heads at the same time, because the heads to not travel the
same path on their respective surfaces at the same time. There's no
real vertical alignment of the tracks between surfaces, and every
surface has its own embedded position information that is used when
that surface's head is active. There were attempts at multi-actuator
designs with separate servo arms and multiple channels, but
mechanically they're too difficult to manufacture at high yields as I
understood it.
Perhaps a stupid question, but why don't they read from all platters in
parallel?
The answer is in the text you quoted above.
There are drives now with two level actuators.
The primary actuator is the standard actuator you are familiar with
which moves all the arms.
The secondary actuator is a piezo crystal towards the head end of the
arm which can move the head a few tracks very quickly without having to
move the arm, and these are one per head. In theory, this might allow
multiple heads to lock on to their respective tracks at the same time
for parallel reads, but I haven't heard that they are used in this way.
If you go back to the late 1970's before tracks had embedded servo data,
on multi-platter disks you had one surface which contained the head
positioning servo data, and the drive relied on accurate vertical
alignment between heads/surfaces to keep on track (and drives could
head-switch instantly). Around 1980, tracks got too close together for
this to work anymore, and the servo positioning data was embedded into
each track itself. The very first drives of this type scanned all the
surfaces on startup to build up an internal table of the relative
misalignment of tracks across the surfaces, but this rapidly became
unviable as drive capacity increased and this scan would take an
unreasonable length of time. It may be that modern drives learn this as
they go - I don't know.
--
Andrew Gabriel
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