As solid state drives based on flash technology are becoming a staple for persistent data storage in data centers, it is important to understand their reliability characteristics. While there is a large body of work based on experiments with individual flash chips in a controlled lab environment under synthetic workloads, there is a dearth of information on their behavior in the field. This paper provides a large-scale field study covering many millions of drive days, ten different drive models, different flash technologies (MLC, eMLC, SLC) over 6 years of production use in Google’s data centers. We study a wide range of reliability characteristics and come to a number of unexpected conclusions. For example, raw bit error rates (RBER) grow at a much slower rate with wear-out than the exponential rate commonly assumed and, more importantly, they are not predictive of uncorrectable errors or other error modes. The widely used metric UBER (uncorrectable bit error rate) is not a meaningful metric, since we see no correlation between the number of reads and the number of uncorrectable errors. We see no evidence that higher-end SLC drives are more reliable than MLC drives within typical drive lifetimes. Comparing with traditional hard disk drives, flash drives have a significantly lower replacement rate in the field, however, they have a higher rate of uncorrectable errors.