One of our customers read the Google study in the previous post and sent along this link and the conclusion is fascinating. I certainly hope that all our customers will read this . You can download it here

Disk failures in the real world:
What does an MTTF of 1,000,000 hours mean to you?

Bianca Schroeder Garth A. Gibson
Computer Science Department
Carnegie Mellon University
{bianca, garth}

7 Conclusion

Many have pointed out the need for a better understanding of what disk failures look like in the field. Yet hardly any published work exists that provides a large-scale study of disk failures in production systems. As a first step towards closing this gap, we have analyzed disk replacement data from a number of large production systems, spanning more than 100,000 drives from at least four different vendors, including drives with SCSI, FC and SATA interfaces. Below is a summary of a few of our results.

* Large-scale installation field usage appears to differ widely from nominal datasheet MTTF conditions. The field replacement rates of systems were significantly larger than we expected based on datasheet MTTFs.

* For drives less than five years old, field replacement rates were larger than what the datasheet MTTF suggested by a factor of 2-10. For five to eight year old drives, field replacement rates were a factor of 30 higher than what the datasheet MTTF suggested.

* Changes in disk replacement rates during the first five years of the lifecycle were more dramatic than often assumed. While replacement rates are often expected to be in steady state in year 2-5 of operation (bottom of the “bathtub curve”), we observed a continuous increase in replacement rates, starting as early as in the second year of operation.

* In our data sets, the replacement rates of SATA disks are not worse than the replacement rates of SCSI or FC disks. This may indicate that disk-independent factors, such as operating conditions, usage and environmental factors, affect replacement rates more than component specific factors. However, the only evidence we have of a bad batch of disks was found in a collection of SATA disks experiencing high media error rates. We have too little data on bad batches to estimate the relative frequency of bad batches by type of disk, although there is plenty of anecdotal evidence that bad batches are not unique to SATA disks.

* The common concern that MTTFs underrepresent infant mortality has led to the proposal of new standards that incorporate infant mortality [33]. Our findings suggest that the underrepresentation of the early onset of wear-out is a much more serious factor than underrepresentation of infant mortality and recommend to include this in new standards.

* While many have suspected that the commonly made assumption of exponentially distributed time between failures/replacements is not realistic, previous studies have not found enough evidence to prove this assumption wrong with significant statistical confidence [8]. Based on our data analysis, we are able to reject the hypothesis of exponentially distributed time between disk replacements with high confidence. We suggest that researchers and designers use field replacement data, when possible, or two parameter distributions, such as the Weibull distribution.

* We identify as the key features that distinguish the empirical distribution of time between disk replacements from the exponential distribution, higher levels of variability and decreasing hazard rates. We find that the empirical distributions are fit well by a Weibull distribution with a shape parameter between 0.7 and 0.8.

* We also present strong evidence for the existence of correlations between disk replacement interarrivals. In particular, the empirical data exhibits significant levels of autocorrelation and long-range dependence.

I wonder why these folks are not invited to speak at storage conferences like SNW ? Could it be that the vendor community has something to hide?

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