Figure 2 presents the average Annualized Failure Rates
(AFR) for all drives in our study, aged zero to 5 years,
and is derived from our older repairs database. The data
are broken down by the age a drive was when it failed.
Note that this implies some overlap between the sample
sets for the 3-month, 6-month, and 1-year ages, because
a drive can reach its 3-month, 6-month and 1-year age
all within the observation period. Beyond 1-year there is
no more overlap.
While it may be tempting to read this graph as strictly
failure rate with drive age, drive model factors are
strongly mixed into these data as well. We tend to source
a particular drive model only for a limited time (as new,
more cost-effective models are constantly being introduced),
so it is often the case that when we look at sets
of drives of different ages we are also looking at a very
different mix of models. Consequently, these data are
not directly useful in understanding the effects of disk
age on failure rates (the exception being the first three
data points, which are dominated by a relatively stable
mix of disk drive models). The graph is nevertheless a
good way to provide a baseline characterization of failures
across our population. It is also useful for later
studies in the paper, where we can judge how consistent
the impact of a given parameter is across these diverse
drive model groups. A consistent and noticeable impact
across all groups indicates strongly that the signal being
measured has a fundamentally powerful correlation with
failures, given that it is observed across widely varying
ages and models.
The observed range of AFRs (see Figure 2) varies
from 1.7%, for drives that were in their first year of operation,
to over 8.6%, observed in the 3-year old popFigure
2: Annualized failure rates broken down by age groups
ulation. The higher baseline AFR for 3 and 4 year old
drives is more strongly influenced by the underlying reliability
of the particular models in that vintage than by
disk drive aging effects. It is interesting to note that our
3-month, 6-months and 1-year data points do seem to
indicate a noticeable influence of infant mortality phenomena,
with 1-year AFR dropping significantly