NAND Flash a high-density, low-cost storage body

This paper first introduces some existing kinds of bad block management (BBM) methods, by analyzing the NAND storage table of typical embedded system, points out the advantages of lightweight management method, analyzes the lightweight management method which is widely used at present, points out the defects and explains the improvement method in detail.

NAND http://www.aliyun.com/zixun/aggregation/12592.html ">flash is a high-density and low-cost storage, it has been widely used in a variety of embedded systems, USB storage devices, SD cards, Devices such as mobile phones, cameras and solid-state drives use NAND chips. Its internal structure is organized according to block/page, a NAND chip contains several blocks, and the interior of the block is composed of several pages. NAND chip factory may contain a number of bad blocks, in the use of the process may also produce new bad blocks, when a block is marked as a bad block, you should not write data to avoid data loss. Because of the limited number of NAND erase, and will be used in the process of generating new bad blocks, the general need for additional software or hardware to match it to use.

The classification of NAND bad block management methods

At present, NAND bad block management methods can be divided into the following categories:

It uses an additional FTL (Flash translation Layer) chip to manage NAND, shielding bad block information from the outside, USB disk, SD card, MMC card and SSD drive all use this management method. This simplifies the NAND operation, but also the bad block of information is not visible to the outside, if the system appears to be a problem related to the block, positioning and debugging becomes difficult, in addition, the FTL chip also requires additional hardware costs.

JFFS2, YAFFS2, flashfx these file systems specifically for NAND can manage bad blocks.

Some middleware (middleware) is dedicated to NAND management, such as UBI.

NAND-managed hardware or software modules not only provide bad block management, but also support the load averaging of NAND erase operations. Lightweight bad block management focuses only on bad blocks, does not provide average support for the scratch load, and does not depend on any third party libraries. Therefore, lightweight bad block management reduces the complexity of the system and eliminates the time to load the file system or initialize the middleware, which is widely used in embedded systems.

Figure 1 shows the content layout of NAND internals in several typical embedded systems. If you need to write a variety of data to NAND frequently, it is best to use NAND file system or NAND management middleware to manage the areas that need to be written. Areas that rarely need to be updated, such as bootloader, VPD, and Kernel, require lightweight, bad-block management without load averaging. In many embedded systems, the amount of data that needs to be written to NAND is small, and the frequency is low, such as routers, printers, and PLC, which can be used only for lightweight, bad-block management.

Fig. 1. Nand Memory Map of a typical embedded system