Introduction

Over-provisioning (sometimes spelled as OP, over provisioning, or overprovisioning) is the difference between the physical capacity of the Flash memory and the logical capacity presented through the operating system (OS) as available for the user. It is a term used to describe the process of increasing the spare area on a flash memory card/drive. In short, over-provisioning means that more physical NAND flash is present than what the system can access at any time. This document provides details of the Dell’s over-provisioned MLC NAND based SD Card.

Over-provisioning increases the endurance and performance of the flash memory card/drive and decreases the flash memory wear and helps lower the write amplification when the flash memory controller writes to the flash memory.

To mitigate this erase-before-write effect, over provisioning is used by Dell to provide pre-erased blocks. Over provisioning allows for the direct writing of data into the over provisioned or “hidden” blocks of space in the foreground operations. In the background, a cleanup routine of moving the data from the “hidden” area into the user area occurs. This cleanup process of freeing up and erasing blocks is managed to ensure that most writes to the drive do not require the slower erase step prior to writing to the sector. Having more “hidden” capacity in “free” blocks available to the SD card allows for better write performance.

Dell’s 8GB and 16GB vFlash SD Card

As of Dec 2011, Dell released high endurance 8GB (Giga Byte) and 16GB vFlash SD cards based on MLC NAND flash. These cards are 100% factory overprovisioned for highest endurance.

The 8 GB vFlash SD card is built with two 64 Gbit NAND flash chips totaling physical NAND capacity of 16 GB. It has 8GB more physical NAND flash than the logical capacity presented through the operating system (OS) as available for the user. The extra 8 GB NAND blocks are reserved for bad block replacement in the case of NAND block failure.

The 16 GB vFlash SD card is built with two 128 Gbit NAND flash chips totaling physical NAND capacity of 32 GB. It has 16GB more physical NAND flash than the logical capacity presented through the operating system (OS) as available for the user. The extra 16 GB NAND blocks are reserved for bad block replacement in the case of NAND block failure.