[ltp] Re: Making hard drive stop when not in use

[Theodore Ts'o]

On Fri, Oct 01, 2004 at 05:48:32PM -0500, Rob Browning wrote:
> Juergen Stuber <juergen@jstuber.net> writes:
> 
> > it is very easy to configure so that it also spins down the harddisk
> > when on AC, I also do it to prevent the drives from wearing out.
> 
> Hmm.  I'd gotten the impression that it was starts/stops (and head
> parks/unparks) that were the primary wear on the drive, not continuous
> spin.

It's both, actually.  There are a couple of things that can cause wear
on a drive, thus shortening its life:

1.  Landing/takeoff cycles on the drive (which happen every time the
drive is spun up or down).  The disk drive heads are pushed down
towards the patters by springs, and when the disk starts spinning, the
head drags along the platter until the disk is going fast enough that
an air cushion is established that aerodynamically pushes the hard
drive heads away from the platter.  When the drive stops spinning, the
air cushion disappears and the springs push the head towards the
platter.  The probability of failure caused by wear and tear on the
hard drive dragging across the surface of the drive increases
geometrically with each landing/takeoff.  Modern drives are generally
engineered such that P_failure reaches 50% after 50,000
landing/takeoff cycles.

2.  The life of the motor.  The motor/ball bearings on some hard
drives may not be engineered for continuous operation.  So if the disk
is spinning continuously, this may also shorten the life of the drive.
In particular, some desktop drives and most laptop drives are *not*
engineered for continuous operation.  Desktop drives because the hard
drive manufacturers are trying to save pennies on each drive so they
can make a profit even when Wal-Mart and Dell are forcing them to
lower prices every other day.  Laptop drives because they are
constrained by power and space limitations.

3.  Thermal issues.  Some hard drives are installed without sufficient
heat sinking so they can withstand being operated continuously.  A
good example of this is the hard drive in the iPod.


The bottom line is that any time you use the hard drive, you will be
causing wear and tear, whether it is by spinning the hard drive or by
causing the heads to take off and land repeatedly.  Different hard
drives are engineered with different lifetimes in mind, but
ultimately, hard drives do not last forever.  2-3 years if you're
lucky, and if you're truly paranoid, and given how cheap hard drives
are, upgrading them with a newer, bigger model every 18-24 months is
not a bad idea.

						- Ted