Is DWPD Still a Useful SSD Spec?

Summary

For those not already familiar, DWPD is an initialism for (total) Drive Writes Per Day, and is intended as a specification for SSD write endurance.With very few exceptions, all solid-state drives currently available for purchase are based on NAND flash. At the very lowest level, they work by charging an individual cell to indicate that it stores a 1, or discharging that cell to indicate that it stores a 0.(If you’re already bristling because we’ve just described SLC flash and most modern drives are TLC or higher, don’t fret–we’ll get to that in a bit.)Compared to mechanical hard drives, NAND flash SSDs offer incredibly increased IOPS, lower latency, greater resistance to fragmentation-related performance issues, and more. They do, however, introduce a few new problems, one of which is limited write endurance.We don’t think we can really do the DWPD question justice without covering a fair bit of information on the hows and whys of drive failures first–so buckle up!Fail DifferentWith mechanical hard drives, failure is typically not due to degradation of the media itself–it is more commonly either a mechanical issue (such as a seized bearing or a crashed head), an electrical issue (such as a motor failure due to a short in the windings), or a controller failure.Although SSDs largely do away with both the mechanical and electrical issues, they are still quite subject to controller failure–and they also introduce a couple of new potential issues with the media itself.As the NAND flash media ages, it becomes both slower to charge and more difficult to accurately charge to a precise desired voltage. In addition, NAND flash cells cannot maintain a charge unaided forever–the voltage in a charged cell slowly decreases over time, which can change the apparent stored value in that cell–and age and use can also cause this problem to accelerate.It may be helpful at this point to list and briefly describe common failure modes and their symptoms.Mechanical HDD Failure ModesFor the...