The use of solid state drives based on NAND flash technology is continuously growing. As more data either lives on flash or is being cached on flash, data durability and availability critically depend on flash reliability. This paper provides a detailed field study of flash reliability based on data collected over 6 years in a large-scale data center production environment. The data spans many millions of drive days, ten different drive models, different flash technologies (MLC and SLC) and feature sizes (ranging from 24nm to 50nm). The paper analyses this data in order to derive a better understanding of flash reliability in the field, including the most prevalent types of errors and hardware failures and their frequency, and how different factors impact flash reliability.