While every product has a life expectancy, not every industry acknowledges this up
front! Due to the incredibly precise nature of the technology used in hard disks (we're
talking about millionths of an inch in many regards) and the fact that mechanical
components are used, it is impossible to guarantee the reliability of even the
highest-quality disks for more than a few years. Hard disks follow the so-called bathtub curve model of failures: they have
a relatively high rate of "infant mortality" (or early failure), a period of
very low failures for several years, and then a gradual reduction in reliability as they
reach the end of their useful life. To ensure that those who rely on hard drives do not
continue to use them as they enter the period of increased failure that results from
component wear-out, every hard disk has defined as one of its specifications the
manufacturer's intended service life for the product.
The service life of a modern hard disk is usually about three to five years. In my
opinion, what the manufacturer is telling you with this number is this: "If your
drive gives you good service for this length of time without any trouble, you've gotten
good value from it and you should consider getting a new drive if reliability is important
to you; the drive isn't going to fail the instant it hits this age, but the chances of it
failing will increase significantly the longer you use it past this point". This
number is in many ways more important than the "impressive" MTBF numbers that are so eagerly thrown around, and is in fact
intended to be used in conjunction with MTBF to present a realistic picture of drive life
and reliability.
Interestingly, the claimed service life is often longer than the warranty period for
the drive. For example, the service life might be five years but the warranty period only
three years. Think about what this means. ;^) Basically, it says that the manufacturer
thinks the drive should last five years, but they aren't going to bet on it lasting
more than three! I personally think the warranty
period is a better indication of a drive's true useful life--to a point--because the
warranty period is where the manufacturer "puts their money where their mouth
is". Sometimes warranty periods are made longer for marketing reasons, but no
manufacturer will warrant a product for three years if they expect to see significant
problems in that time, because they will lose their shirts.
If reliability is important, you may want to make sure you get a new hard disk before
your warranty period is up, or at the very least, before the drive reaches the end of its
service life. Of course, at the rate technology changes, in
three years you will probably want a new disk for performance reasons as well. Oh, and
it goes without saying that anyone who cares about hard disk reliability should be
performing regular backups.
The real world life of a hard disk is almost always going to be significantly higher
than the stated service life, which tends to be very conservative. One reasons why is that
even if the warranty doesn't cover the entire stated service life, most companies don't
want to see their drives failing within the period of time that they say they won't--it
looks bad. Another reason is because the service life only represents when the odds
of failure increase; there is still a statistical distribution of actual failures. I have
used many hard disks that have been in operation for 10 or even 12 years, and these are
older technology drives that never had anywhere near the stated reliability levels of
today's disks. So there certainly is no reason to believe that the day after your drive's
third anniversary, you need to yank it and replace it with a new one. But just remember
that the longer you go past the drive's service life, the more the chances of a failure
increase. The environmental conditions, and how the drive is used, will also have an
important impact on its overall life.
Next: Start/Stop Cycles
