Technology placed around the NAND by the manufacturer can change endurance as well, for better or worse. Thankfully, SSD endurance isn’t set by P/E cycle limits alone. Error Correction, Overprovisioning, and Firmware This decrease in cycles is obviously a bad thing for endurance. As the industry transitions from Multi Level Cell (MLC) to Triple Level Cell (TLC) SSDs, which store 3 bits per cell, the available P/E cycles decrease. These cycles occur whenever existing data needs to be overwritten in a flash cell. SSD endurance is limited because the NAND flash that powers SSDs has a finite number of “program/erase” (P/E) cycles before it can’t be used anymore. The physics of SSD endurance are complicated, but the results are simple: SSDs wear out as you write to them. Choose the wrong SSD endurance and you’ll end up replacing the drive early or overpaying for a higher endurance drive than needed. SSD endurance is the total amount of data that an SSD is guaranteed to be able to write under warranty, often specified in “TBW” or “DWPD” (which we’ll discuss a little later). There’s one more choice you need to make, and it’s a choice you might not have had to make before: the SSD endurance level. You also need to choose the right capacity, of course, anywhere from 100s of gigabytes to multiple-terabytes. You need to select from three major, incompatible interfaces ( SATA, SAS, or NVMe™). You have to pick the right the form factor so that the drive will fit in your server. Choosing the right SSD is a complicated process, after all. You’ve probably looked at an SSD datasheet and been a little overwhelmed. In this post I’ll discuss SSD endurance and how this affects your choice of SSD, plus give you some rules of thumb for making the right choice. My first post discussed the role of latency in storage architectures. This is my second post in the “Speeds, Feeds, and Needs” blog series, designed to explain the more technical elements of enterprise storage in terms that are understandable to everyone.
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