Appears in the Proceedings of the 5th USENIX Conference on File and Storage Technologies (FAST’07), February 2007
Failure Trends in a Large Disk Drive Population
Eduardo Pinheiro, Wolf-Dietrich Weber and Luiz Andr´e Barroso
Google Inc.
1600 Amphitheatre Pkwy
Mountain View, CA 94043
{edpin,wolf,luiz}@google.com
Abstract
It is estimated that over 90% of all new information produced
in the world is being stored on magnetic media, most of it on
hard disk drives. Despite their importance, there is relatively
little published work on the failure patterns of disk drives, and
the key factors that affect their lifetime. Most available data
are either based on extrapolation from accelerated aging experiments
or from relatively modest sized field studies. Moreover,
larger population studies rarely have the infrastructure in place
to collect health signals from components in operation, which
is critical information for detailed failure analysis.
We present data collected from detailed observations of a
large disk drive population in a production Internet services deployment.
The population observed is many times larger than
that of previous studies. In addition to presenting failure statistics,
we analyze the correlation between failures and several
parameters generally believed to impact longevity.
Our analysis identifies several parameters from the drive’s
self monitoring facility (SMART) that correlate highly with
failures. Despite this high correlation, we conclude that models
based on SMART parameters alone are unlikely to be useful
for predicting individual drive failures. Surprisingly, we found
that temperature and activity levels were much less correlated
with drive failures than previously reported.
1 Introduction
The tremendous advances in low-cost, high-capacity
magnetic disk drives have been among the key factors
helping establish a modern society that is deeply reliant
on information technology. High-volume, consumergrade
disk drives have become such a successful product
that their deployments range from home computers
and appliances to large-scale server farms. In 2002, for
example, it was estimated that over 90% of all new information
produced was stored on magnetic media, most
of it being hard disk drives [12]. It is therefore critical
to improve our understanding of how robust these components
are and what main factors are associated with
failures. Such understanding can be particularly useful
for guiding the design of storage systems as well as devising
deployment and maintenance strategies.
Despite the importance of the subject, there are very
few published studies on failure characteristics of disk
drives. Most of the available information comes from
the disk manufacturers themselves [2]. Their data are
typically based on extrapolation from accelerated life
test data of small populations or from returned unit
databases. Accelerated life tests, although useful in providing
insight into how some environmental factors can
affect disk drive lifetime, have been known to be poor
predictors of actual failure rates as seen by customers
in the field [7]. Statistics from returned units are typically
based on much larger populations, but since there
is little or no visibility into the deployment characteristics,
the analysis lacks valuable insight into what actually
happened to the drive during operation. In addition,
since units are typically returned during the warranty period
(often three years or less), manufacturers’ databases
may not be as helpful for the study of long-term effects.
A few recent studies have shed some light on field
failure behavior of disk drives [6, 7, 9, 16, 17, 19, 20].
However, these studies have either reported on relatively
modest populations or did not monitor the disks closely
enough during deployment to provide insights into the
factors that might be associated with failures.
Disk drives are generally very reliable but they are
also very complex components. This combination
means that although they fail rarely, when they do fail,
the possible causes of failure can be numerous. As a
result, detailed studies of very large populations are the
only way to collect enough failure statistics to enable
meaningful conclusions. In this paper we present one
such study by examining the population of hard drives
under deployment within Google’s computing infras-
tructure.
We have built an infrastructure that collects vital information
about all Google’s systems every few minutes,
and a repository that stores these data in timeseries
format (essentially forever) for further analysis.
The information collected includes environmental factors
(such as temperatures), activity levels and many of
the Self-Monitoring Analysis and Reporting Technology
(SMART) parameters that are believed to be good indicators
of disk drive health. We mine through these data
and attempt to find evidence that corroborates or contradicts
many of the commonly held beliefs about how
various factors can affect disk drive lifetime.
Our paper is unique in that it is based on data from a
disk population size that is typically only available from
vendor warranty databases, but has the depth of deployment
visibility and detailed lifetime follow-up that only
an end-user study can provide. Our key findings are:
• Contrary to previously reported results, we found
very little correlation between failure rates and either
elevated temperature or activity levels.
• Some SMART parameters (scan errors, reallocation
counts, offline reallocation counts, and probational
counts) have a large impact on failure proba-
bility.
• Given the lack of occurrence of predictive SMART
signals on a large fraction of failed drives, it is unlikely
that an accurate predictive failure model can
be built based on these signals alone.
2 Background
In this section we describe the infrastructure that was
used to gather and process the data used in this study,
the types of disk drives included in the analysis, and information
on how they are deployed.
2.1 The System Health Infrastructure
The System Health infrastructure is a large distributed
software system that collects and stores hundreds of
attribute-value pairs from all of Google’s servers, and
provides the interface for arbitrary analysis jobs to process
that data.
The architecture of the System Health infrastructure
is shown in Figure 1. It consists of a data collection
layer, a distributed repository and an analysis framework.
The collection layer is responsible for getting information
from each of thousands of individual servers
into a centralized repository. Different flavors of collectors
exist to gather different types of data. Much of
the health information is obtained from the machines directly.
A daemon runs on every machine and gathers
local data related to that machine’s health, such as environmental
parameters, utilization information of various
Figure 1: Collection, storage, and analysis architecture.
resources, error indications, and configuration information.
It is imperative that this daemon’s resource usage
be very light, so not to interfere with the applications.
One way to assure this is to have the machine-level collector
poll individual machines relatively infrequently
(every few minutes). Other slower changing data (such
as configuration information) and data from other existing
databases can be collected even less frequently than
that. Most notably for this study, data regarding machine
repairs and disk swaps are pulled in from another
database.
The System Health database is built upon Bigtable
[3], a distributed data repository widely used within
Google, which itself is built upon the Google File System
(GFS) [8]. Bigtable takes care of all the data layout,
compression, and access chores associated with a large
data store. It presents the abstraction of a 2-dimensional
table of data cells, with different versions over time making
up a third dimension. It is a natural fit for keeping
track of the values of different variables (columns) for
different machines (rows) over time. The System Health
database thus retains a complete time-ordered history of
the environment, utilization, error, configuration, and repair
events in each machine’s life.
Analysis programs run on top of the System Health
database, looking at information from individual machines,
or mining the data across thousands of machines.
Large-scale analysis programs are typically built upon
Google’s Mapreduce [5] framework. Mapreduce automates
the mechanisms of large-scale distributed compu-
tation (such as work distribution, load balancing, tolerance
of failures), allowing the user to focus simply on
the algorithms that make up the heart of the computa-
tion.
The analysis pipeline used for this study consists of
a Mapreduce job written in the Sawzall language and
framework [15] to extract and clean up periodic SMART
data and repair data related to disks, followed by a pass
through R [1] for statistical analysis and final graph gen-
eration.
2.2 Deployment Details
The data in this study are collected from a large number
of disk drives, deployed in several types of systems
across all of Google’s services. More than one hundred
thousand disk drives were used for all the results presented
here. The disks are a combination of serial and
parallel ATA consumer-grade hard disk drives, ranging
in speed from 5400 to 7200 rpm, and in size from 80 to
400 GB. All units in this study were put into production
in or after 2001. The population contains several models
from many of the largest disk drive manufacturers and
from at least nine different models. The data used for
this study were collected between December 2005 and
August 2006.
As is common in server-class deployments, the disks
were powered on, spinning, and generally in service for
essentially all of their recorded life. They were deployed
in rack-mounted servers and housed in professionallymanaged
datacenter facilities.
Before being put into production, all disk drives go
through a short burn-in process, which consists of a
combination of read/write stress tests designed to catch
many of the most common assembly, configuration, or
component-level problems. The data shown here do not
include the fall-out from this phase, but instead begin
when the systems are officially commissioned for use.
Therefore our data should be consistent with what a regular
end-user should see, since most equipment manufacturers
put their systems through similar tests before
shipment.
2.3 Data Preparation
Definition of Failure. Narrowly defining what constitutes
a failure is a difficult task in such a large operation.
Manufacturers and end-users often see different
statistics when computing failures since they use different
definitions for it. While drive manufacturers often
quote yearly failure rates below 2% [2], user studies have
seen rates as high as 6% [9]. Elerath and Shah [7] report
between 15-60% of drives considered to have failed at
the user site are found to have no defect by the manufacturers
upon returning the unit. Hughes et al. [11] observe
between 20-30% “no problem found” cases after
analyzing failed drives from their study of 3477 disks.
From an end-user’s perspective, a defective drive is
one that misbehaves in a serious or consistent enough
manner in the user’s specific deployment scenario that
it is no longer suitable for service. Since failures are
sometimes the result of a combination of components
(i.e., a particular drive with a particular controller or cable,
etc), it is no surprise that a good number of drives
that fail for a given user could be still considered operational
in a different test harness. We have observed
that phenomenon ourselves, including situations where
a drive tester consistently “green lights” a unit that invariably
fails in the field. Therefore, the most accurate
definition we can present of a failure event for our study
is: a drive is considered to have failed if it was replaced
as part of a repairs procedure. Note that this definition
implicitly excludes drives that were replaced due to an
upgrade.
Since it is not always clear when exactly a drive failed,
we consider the time of failure to be when the drive was
replaced, which can sometimes be a few days after the
observed failure event. It is also important to mention
that the parameters we use in this study were not in use
as part of the repairs diagnostics procedure at the time
that these data were collected. Therefore there is no risk
of false (forced) correlations between these signals and
repair outcomes.
Filtering. With such a large number of units monitored
over a long period of time, data integrity issues invariably
show up. Information can be lost or corrupted along
our collection pipeline. Therefore, some cleaning up of
the data is necessary. In the case of missing values, the
individual values are marked as not available and that
specific piece of data is excluded from the detailed studies.
Other records for that same drive are not discarded.
In cases where the data are clearly spurious, the entire
record for the drive is removed, under the assumption
that one piece of spurious data draws into question other
fields for the same drive. Identifying spurious data, however,
is a tricky task. Because part of the goal of studying
the data is to learn what the numbers mean, we must be
careful not to discard too much data that might appear
invalid. So we define spurious simply as negative counts
or data values that are clearly impossible. For example,
some drives have reported temperatures that were
hotter than the surface of the sun. Others have had negative
power cycles. These were deemed spurious and
removed. On the other hand, we have not filtered any
suspiciously large counts from the SMART signals, under
the hypothesis that large counts, while improbable as
raw numbers, are likely to be good indicators of something
really bad with the drive. Filtering for spurious
values reduced the sample set size by less than 0.1%.
3 Results
We now analyze the failure behavior of our fleet of disk
drives using detailed monitoring data collected over a
nine-month observation window. During this time we
recorded failure events as well as all the available environmental
and activity data and most of the SMART
parameters from the drives themselves. Failure information
spanning a much longer interval (approximately five
years) was also mined from an older repairs database.
All the results presented here were tested for their statistical
significance using the appropriate tests.
3.1 Baseline Failure Rates
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 from
the AFR observed in the first three months.
3.2 Manufacturers, Models, and Vintages
Failure rates are known to be highly correlated with drive
models, manufacturers and vintages [18]. Our results do
not contradict this fact. For example, Figure 2 changes
significantly when we normalize failure rates per each
drive model. Most age-related results are impacted by
drive vintages. However, in this paper, we do not show a
breakdown of drives per manufacturer, model, or vintage
due to the proprietary nature of these data.
Interestingly, this does not change our conclusions. In
contrast to age-related results, we note that all results
shown in the rest of the paper are not affected significantly
by the population mix. None of our SMART data
results change significantly when normalized by drive
model. The only exception is seek error rate, which is
dependent on one specific drive manufacturer, as we discuss
in section 3.5.5.
3.3 Utilization
The literature generally refers to utilization metrics by
employing the term duty cycle which unfortunately has
no consistent and precise definition, but can be roughly
characterized as the fraction of time a drive is active out
of the total powered-on time. What is widely reported in
the literature is that higher duty cycles affect disk drives
negatively [4, 21].
It is difficult for us to arrive at a meaningful numerical
utilization metric given that our measurements do
not provide enough detail to derive what 100% utilization
might be for any given disk model. We choose instead
to measure utilization in terms of weekly averages
of read/write bandwidth per drive. We categorize utilization
in three levels: low, medium and high, corresponding
respectively to the lowest 25th percentile, 50-75th
percentiles and top 75th percentile. This categorization
is performed for each drive model, since the maximum
bandwidths have significant variability across drive families.
We note that using number of I/O operations and
bytes transferred as utilization metrics provide very similar
results. Figure 3 shows the impact of utilization on
AFR across the different age groups.
Overall, we expected to notice a very strong and consistent
correlation between high utilization and higher
failure rates. However our results appear to paint a more
complex picture. First, only very young and very old
age groups appear to show the expected behavior. After
the first year, the AFR of high utilization drives is
at most moderately higher than that of low utilization
drives. The three-year group in fact appears to have the
opposite of the expected behavior, with low utilization
drives having slightly higher failure rates than high utilization
ones.
One possible explanation for this behavior is the survival
of the fittest theory. It is possible that the failure
modes that are associated with higher utilization are
more prominent early in the drive’s lifetime. If that is the
case, the drives that survive the infant mortality phase
are the least susceptible to that failure mode, and result
in a population that is more robust with respect to variations
in utilization levels.
Another possible explanation is that previous observations
of high correlation between utilization and failures
has been based on extrapolations from manufacturers’
accelerated life experiments. Those experiments are
likely to better model early life failure characteristics,
and as such they agree with the trend we observe for the
young age groups. It is possible, however, that longer
term population studies could uncover a less pronounced
effect later in a drive’s lifetime.
When we look at these results across individual models
we again see a complex pattern, with varying patterns
of failure behavior across the three utilization levels.
Taken as a whole, our data indicate a much weaker
correlation between utilization levels and failures than
previous work has suggested.
Figure 3: Utilization AFR
3.4 Temperature
Temperature is often quoted as the most important environmental
factor affecting disk drive reliability. Previous
studies have indicated that temperature deltas as low as
15C can nearly double disk drive failure rates [4]. Here
we take temperature readings from the SMART records
every few minutes during the entire 9-month window
of observation and try to understand the correlation between
temperature levels and failure rates.
We have aggregated temperature readings in several
different ways, including averages, maxima, fraction of
time spent above a given temperature value, number of
times a temperature threshold is crossed, and last temperature
before failure. Here we report data on averages
and note that other aggregation forms have shown similar
trends and and therefore suggest the same conclu-
sions.
We first look at the correlation between average temperature
during the observation period and failure. Figure
4 shows the distribution of drives with average temperature
in increments of one degree and the corresponding
annualized failure rates. The figure shows that failures
do not increase when the average temperature increases.
In fact, there is a clear trend showing that lower
temperatures are associated with higher failure rates.
Only at very high temperatures is there a slight reversal
of this trend.
Figure 5 looks at the average temperatures for different
age groups. The distributions are in sync with Figure
4 showing a mostly flat failure rate at mid-range temperatures
and a modest increase at the low end of the temperature
distribution. What stands out are the 3 and 4year
old drives, where the trend for higher failures with
higher temperature is much more constant and also more
pronounced.
Overall our experiments can confirm previously re-
Figure 4: Distribution of average temperatures and failures
rates.
Figure 5: AFR for average drive temperature.
ported temperature effects only for the high end of our
temperature range and especially for older drives. In the
lower and middle temperature ranges, higher temperatures
are not associated with higher failure rates. This is
a fairly surprising result, which could indicate that datacenter
or server designers have more freedom than previously
thought when setting operating temperatures for
equipment that contains disk drives. We can conclude
that at moderate temperature ranges it is likely that there
are other effects which affect failure rates much more
strongly than temperatures do.
3.5 SMART Data Analysis
We now look at the various self-monitoring signals that
are available from virtually all of our disk drives through
the SMART standard interface. Our analysis indicates
that some signals appear to be more relevant to the study
of failures than others. We first look at those in detail,
and then list a summary of our findings for the remaining
ones. At the end of this section we discuss our results
and reason about the usefulness of SMART parameters
in obtaining predictive models for individual disk drive
failures.
We present results in three forms. First we compare
the AFR of drives with zero and non-zero counts for a
given parameter, broken down by the same age groups
as in figures 2 and 3. We also find it useful to plot the
probability of survival of drives over the nine-month observation
window for different ranges of parameter values.
Finally, in addition to the graphs, we devise a single
metric that could relay how relevant the values of
a given SMART parameter are in predicting imminent
failures. To that end, for each SMART parameter we
look for thresholds that increased the probability of failure
in the next 60 days by at least a factor of 10 with
respect to drives that have zero counts for that parameter.
We report such Critical Thresholds whenever we are
able to find them with high confidence (> 95%).
3.5.1 Scan Errors
Drives typically scan the disk surface in the background
and report errors as they discover them. Large scan error
counts can be indicative of surface defects, and therefore
are believed to be indicative of lower reliability. In our
population, fewer than 2% of the drives show scan errors
and they are nearly uniformly spread across various disk
models.
Figure 6 shows the AFR values of two groups of
drives, those without scan errors and those with one or
more. We plot bars across all age groups in which we
have statistically significant data. We find that the group
of drives with scan errors are ten times more likely to fail
than the group with no errors. This effect is also noticed
when we further break down the groups by disk model.
From Figure 8 we see a drastic and quick decrease in
survival probability after the first scan error (left graph).
A little over 70% of the drives survive the first 8 months
after their first scan error. The dashed lines represent the
95% confidence interval. The middle plot in Figure 8
separates the population in four age groups (in months),
and shows an effect that is not visible in the AFR plots. It
appears that scan errors affect the survival probability of
young drives more dramatically very soon after the first
scan error occurs, but after the first month the curve flattens
out. Older drives, however, continue to see a steady
decline in survival probability throughout the 8-month
period. This behavior could be another manifestation of
infant mortality phenomenon. The right graph in figure 8
looks at the effect of multiple scan errors. While drives
with one error are more likely to fail than those with
none, drives with multiple errors fail even more quickly.
Figure 6: AFR for scan errors. Figure 7: AFR for reallocation counts.
Figure 8: Impact of scan errors on survival probability. Left figure shows aggregate survival probability for all drives after first
scan error. Middle figure breaks down survival probability per drive ages in months. Right figure breaks down drives by their
number of scan errors.
The critical threshold analysis confirms what the
charts visually imply: the critical threshold for scan errors
is one. After the first scan error, drives are 39 times
more likely to fail within 60 days than drives without
scan errors.
3.5.2 Reallocation Counts
When the drive’s logic believes that a sector is damaged
(typically as a result of recurring soft errors or a hard error)
it can remap the faulty sector number to a new physical
sector drawn from a pool of spares. Reallocation
counts reflect the number of times this has happened,
and is seen as an indication of drive surface wear. About
9% of our population has reallocation counts greater
than zero. Although some of our drive models show
higher absolute values than others, the trends we observe
are similar across all models.
As with scan errors, the presence of reallocations
seems to have a consistent impact on AFR for all age
groups (Figure 7), even if slightly less pronounced.
Drives with one or more reallocations do fail more often
than those with none. The average impact on AFR
appears to be between a factor of 3-6x.
Figure 11 shows the survival probability after the first
reallocation. We truncate the graph to 8.5 months, due
to a drastic decrease in the confidence levels after that
point. In general, the left graph shows, about 85% of the
drives survive past 8 months after the first reallocation.
The effect is more pronounced (middle graph) for drives
in the age ranges [10,20) and [20, 60] months, while
newer drives in the range [0,5) months suffer more than
their next generation. This could again be due to infant
mortality effects, although it appears to be less drastic in
this case than for scan errors.
After their first reallocation, drives are over 14 times
more likely to fail within 60 days than drives without
reallocation counts, making the critical threshold for this
parameter also one.
Figure 9: AFR for offline reallocation count. Figure 10: AFR for probational count.
Figure 11: Impact of reallocation count values on survival probability. Left figure shows aggregate survival probability for all
drives after first reallocation. Middle figure breaks down survival probability per drive ages in months. Right figure breaks down
drives by their number of reallocations.
3.5.3 Offline Reallocations
Offline reallocations are defined as a subset of the reallocation
counts studied previously, in which only reallocated
sectors found during background scrubbing are
counted. In other words, it should exclude sectors that
are reallocated as a result of errors found during actual
I/O operations. Although this definition mostly holds,
we see evidence that certain disk models do not implement
this definition. For instance, some models show
more offline reallocations than total reallocations. Since
the impact of offline reallocations appears to be significant
and not identical to that of total reallocations, we
decided to present it separately (Figure 9). About 4% of
our population shows non-zero values for offline reallocations,
and they tend to be concentrated on a particular
subset of drive models.
Overall, the effects on survival probability of offline
reallocation seem to be more drastic than those of total
reallocations, as seen in Figure 12 (as before, some
curves are clipped at 8 months because our data for those
points were not within high confidence intervals). Drives
in the older age groups appear to be more highly affected
by it, although we are unable to attribute this effect to
age given the different model mixes in the various age
groups.
After the first offline reallocation, drives have over
21 times higher chances of failure within 60 days than
drives without offline reallocations; an effect that is
again more drastic than total reallocations.
Our data suggest that, although offline reallocations
could be an important parameter affecting failures, it is
particularly important to interpret trends in these values
within specific models, since there is some evidence that
different drive models may classify reallocations differ-
ently.
3.5.4 Probational Counts
Disk drives put suspect bad sectors “on probation” until
they either fail permanently and are reallocated or
continue to work without problems. Probational counts,
Figure 12: Impact of offline reallocation on survival probability. Left figure shows aggregate survival probability for all drives
after first offline reallocation. Middle figure breaks down survival probability per drive ages in months. Right figure breaks down
drives by their number offline reallocation.
Figure 13: Impact of probational count values on survival probability. Left figure shows aggregate survival probability for all
drives after first probational count. Middle figure breaks down survival probability per drive ages in months. Right figure breaks
down drives by their number of probational counts.
therefore, can be seen as a softer error indication. It
could provide earlier warning of possible problems but
might also be a weaker signal, in that sectors on probation
may indeed never be reallocated. About 2% of
our drives had non-zero probational count values. We
note that this number is lower than both online and offline
reallocation counts, likely indicating that sectors
may be removed from probation after further observation
of their behavior. Once more, the distribution of
drives with non-zero probational counts are somewhat
skewed towards a subset of disk drive models.
Figures 10 and 13 show that probational count trends
are generally similar to those observed for offline reallocations,
with age group being somewhat less pronounced.
The critical threshold for probational counts
is also one: after the first event, drives are 16 times more
likely to fail within 60 days than drives with zero probational
counts.
3.5.5 Miscellaneous Signals
In addition to the SMART parameters described in the
previous sections, which we have found to most closely
impact failure rates, we have also studied several other
parameters from the SMART set as well as other environmental
factors. Here we briefly mention our relevant
findings for some of those parameters.
Seek Errors. Seek errors occur when a disk drive fails to
properly track a sector and needs to wait for another revolution
to read or write from or to a sector. Drives report
it as a rate, and it is meant to be used in combination with
model-specific thresholds. When examining our population,
we find that seek errors are widespread within
drives of one manufacturer only, while others are more
conservative in showing this kind of errors. For this one
manufacturer, the trend in seek errors is not clear, changing
from one vintage to another. For other manufacturers,
there is no correlation between failure rates and seek
errors.
CRC Errors. Cyclic redundancy check (CRC) errors
are detected during data transmission between the physical
media and the interface. Although we do observe
some correlation between higher CRC counts and failures,
those effects are somewhat less pronounced. CRC
errors are less indicative of drive failures than that of cables
and connectors. About 2% of our population had
CRC errors.
Power Cycles. The power cycles indicator counts the
number of times a drive is powered up and down. In
a server-class deployment, in which drives are powered
continuously, we do not expect to reach high enough
power cycle counts to see any effects on failure rates.
Our results find that for drives aged up to two years, this
is true, there is no significant correlation between failures
and high power cycles count. But for drives 3 years
and older, higher power cycle counts can increase the
absolute failure rate by over 2%. We believe this is due
more to our population mix than to aging effects. Moreover,
this correlation could be the effect (not the cause)
of troubled machines that require many repair iterations
and thus many power cycles to be fixed.
Calibration Retries. We were unable to reach a consistent
and clear definition of this SMART parameter from
public documents as well as consultations with some of
the disk manufacturers. Nevertheless, our observations
do not indicate that this is a particularly useful parameter
for the goals of this study. Under 0.3% of our drives
have calibration retries, and of that group only about 2%
have failed, making this a very weak and imprecise signal
when compared with other SMART parameters.
Spin Retries. Counts the number of retries when the
drive is attempting to spin up. We did not register a single
count within our entire population.
Power-on hours Although we do not dispute that
power-on hours might have an effect on drive lifetime,
it happens that in our deployment the age of the drive is
an excellent approximation for that parameter, given that
our drives remain powered on for most of their life time.
Vibration This is not a parameter that is part of the
SMART set, but it is one that is of general concern in designing
drive enclosures as most manufacturers describe
how vibration can affect both performance and reliability
of disk drives. Unfortunately we do not have sensor
information to measure this effect directly for drives in
service. We attempted to indirectly infer vibration effects
by considering the differences in failure rates between
systems with a single drive and those with multiple
drives, but those experiments were not controlled
enough for other possible factors to allow us to reach
any conclusions.
3.5.6 Predictive Power of SMART Parameters
Given how strongly correlated some SMART parameters
were found to be with higher failure rates, we were
hopeful that accurate predictive failure models based on
SMART signals could be created. Predictive models are
very useful in that they can reduce service disruption
due to failed components and allow for more efficient
scheduled maintenance processes to replace the less efficient
(and reactive) repairs procedures. In fact, one of
the main motivations for SMART was to provide enough
insight into disk drive behavior to enable such models to
be built.
After our initial attempts to derive such models
yielded relatively unimpressive results, we turned to the
question of what might be the upper bound of the accuracy
of any model based solely on SMART parameters.
Our results are surprising, if not somewhat disappointing.
Out of all failed drives, over 56% of them have no
count in any of the four strong SMART signals, namely
scan errors, reallocation count, offline reallocation, and
probational count. In other words, models based only
on those signals can never predict more than half of the
failed drives. Figure 14 shows that even when we add
all remaining SMART parameters (except temperature)
we still find that over 36% of all failed drives had zero
counts on all variables. This population includes seek
error rates, which we have observed to be widespread in
our population (> 72% of our drives have it) which further
reduces the sample size of drives without any errors.
It is difficult to add temperature to this analysis since
despite it being reported as part of SMART there are no
crisp thresholds that directly indicate errors. However,
if we arbitrarily assume that spending more than 50%
of the observed time above 40C is an indication of possible
problem, and add those drives to the set of predictable
failures, we still are left with about 36% of all
drives with no failure signals at all. Actual useful models,
which need to have small false-positive rates are in
fact likely to do much worse than these limits might sug-
gest.
We conclude that it is unlikely that SMART data alone
can be effectively used to build models that predict failures
of individual drives. SMART parameters still appear
to be useful in reasoning about the aggregate reliability
of large disk populations, which is still very important
for logistics and supply-chain planning. It is possible,
however, that models that use parameters beyond
those provided by SMART could achieve significantly
better accuracies. For example, performance anomalies
and other application or operating system signals could
be useful in conjunction with SMART data to create
more powerful models. We plan to explore this possibility
in our future work.
Figure 14: Percentage of failed drives with SMART errors.
4 Related Work
Previous studies in this area generally fall into two categories:
vendor (disk drive or storage appliance) technical
papers and user experience studies. Disk vendors
studies provide valuable insight into the electromechanical
characteristics of disks and both modelbased
and experimental data that suggests how several
environmental factors and usage activities can affect device
lifetime. Yang and Sun [21] and Cole [4] describe
the processes and experimental setup used by Quantum
and Seagate to test new units and the models that attempt
to make long-term reliability predictions based on accelerated
life tests of small populations. Power-on-hours,
duty cycle, temperature are identified as the key deployment
parameters that impact failure rates, each of them
having the potential to double failure rates when going
from nominal to extreme values. For example, Cole
presents thermal de-rating models showing that MTBF
could degrade by as much as 50% when going from operating
temperatures of 30C to 40C. Cole’s report also
presents yearly failure rates from Seagate’s warranty
database, indicating a linear decrease in annual failure
rates from 1.2% in the first year to 0.39% in the third
(and last year of record). In our study, we did not find
much correlation between failure rate and either elevated
temperature or utilization. It is the most surprising result
of our study. Our annualized failure rates were generally
higher than those reported by vendors, and more consistent
with other user experience studies.
Shah and Elerath have written several papers based
on the behavior of disk drives inside Network Appliance
storage products [6, 7, 19]. They use a reliability
database that includes field failure statistics as well as
support logs, and their position as an appliance vendor
enables them more control and visibility into actual deployments
than a typical disk drive vendor might have.
Although they do not report directly on the correlation
between SMART parameters or environmental factors
and failures (possibly for confidentiality concerns), their
work is useful in enabling a qualitative understanding
of factors what affect disk drive reliability. For example,
they comment that end-user failure rates can be as
much as ten times higher than what the drive manufacturer
might expect [7]; they report in [6] a strong experimental
correlation between number of heads and higher
failure rates (an effect that is also predicted by the models
in [4]); and they observe that different failure mechanisms
are at play at different phases of a drive lifetime
[19]. Generally, our findings are in line with these re-
sults.
User experience studies may lack the depth of insight
into the device inner workings that is possible in manufacturer
reports, but they are essential in understanding
device behavior in real-world deployments. Unfortunately,
there are very few such studies to date, probably
due to the large number of devices needed to observe
statistically significant results and the complex infrastructure
required to track failures and their contributing
factors.
Talagala and Patterson [20] perform a detailed error
analysis of 368 SCSI disk drives over an eighteen
month period, reporting a failure rate of 1.9%. Results
on a larger number of desktop-class ATA drives
under deployment at the Internet Archive are presented
by Schwarz et al [17]. They report on a 2% failure rate
for a population of 2489 disks during 2005, while mentioning
that replacement rates have been as high as 6%
in the past. Gray and van Ingen [9] cite observed failure
rates ranging from 3.3-6% in two large web properties
with 22,400 and 15,805 disks respectively. A recent
study by Schroeder and Gibson [16] helps shed light
into the statistical properties of disk drive failures. The
study uses failure data from several large scale deployments,
including a large number of SATA drives. They
report a significant overestimation of mean time to failure
by manufacturers and a lack of infant mortality effects.
None of these user studies have attempted to correlate
failures with SMART parameters or other environmental
factors.
We are aware of two groups that have attempted
to correlate SMART parameters with failure statistics.
Hughes et al [11, 13, 14] and Hamerly and Elkan [10].
The largest populations studied by these groups was of
3744 and 1934 drives and they derive failure models that
achieve predictive rates as high as 30%, at false positive
rates of about 0.2% (that false-positive rate corresponded
to a number of drives between 20-43% of the
drives that actually failed in their studies). Hughes et al.
also cites an annualized failure rate of 4-6%, based on
their 2-3 month long experiment which appears to use
stress test logs provided by a disk manufacturer.
Our study takes a next step towards a better understanding
of disk drive failure characteristics by essentially
combining some of the best characteristics of studies
from vendor database analysis, namely population
size, with the kind of visibility into a real-world deployment
that is only possible with end-user data.
5 Conclusions
In this study we report on the failure characteristics of
consumer-grade disk drives. To our knowledge, the
study is unprecedented in that it uses a much larger
population size than has been previously reported and
presents a comprehensive analysis of the correlation between
failures and several parameters that are believed to
affect disk lifetime. Such analysis is made possible by
a new highly parallel health data collection and analysis
infrastructure, and by the sheer size of our computing
deployment.
One of our key findings has been the lack of a consistent
pattern of higher failure rates for higher temperature
drives or for those drives at higher utilization levels.
Such correlations have been repeatedly highlighted
by previous studies, but we are unable to confirm them
by observing our population. Although our data do not
allow us to conclude that there is no such correlation,
it provides strong evidence to suggest that other effects
may be more prominent in affecting disk drive reliability
in the context of a professionally managed data center
deployment.
Our results confirm the findings of previous smaller
population studies that suggest that some of the SMART
parameters are well-correlated with higher failure probabilities.
We find, for example, that after their first scan
error, drives are 39 times more likely to fail within 60
days than drives with no such errors. First errors in reallocations,
offline reallocations, and probational counts
are also strongly correlated to higher failure probabilities.
Despite those strong correlations, we find that
failure prediction models based on SMART parameters
alone are likely to be severely limited in their prediction
accuracy, given that a large fraction of our failed drives
have shown no SMART error signals whatsoever. This
result suggests that SMART models are more useful in
predicting trends for large aggregate populations than for
individual components. It also suggests that powerful
predictive models need to make use of signals beyond
those provided by SMART.
Acknowledgments
We wish to acknowledge the contribution of numerous
Google colleagues, particularly in the Platforms and
Hardware Operations teams, who made this study possible,
directly or indirectly; among them: Xiaobo Fan,
Greg Slaughter, Don Yang, Jeremy Kubica, Jim Winget,
Caio Villela, Justin Moore, Henry Green, Taliver Heath,
and Walt Drummond. We are also thankful to our shepherd,
Mary Baker for comments and guidance. A special
thanks to Urs H¨olzle for his extensive feedback on our
drafts.
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