New-Tech Europe | September 2016 | Digital Edition

The figures on the next page depict the concept of wear-leveling. The flash disk in this example has a maximum endurance of 100,000 cycles. Figure 1 represents a disk that does not employ any wear-leveling software, while Figure 2 shows a disk that is managed with wear-leveling schemes. Note that the sectors that have exceeded the 100,000 cycles in Figure 1 are no longer able to store data, and have corrupted the data that they were storing. These sectors represent the high-use areas of the disk. On a FAT disk for example, this may be where the FAT table is stored and corruption here would render the entire file system unreadable. Lower use areas of the disk, which may include application and operating configuration is Multi-Level Cell (MLC) which stores multiple bits per flash cell. This technique results in considerably less endurance, ranging from 10,000 down to 300 for the smallest densities. It also consumes higher power and has lower write speeds, though the flash media is often less expensive. With P/E cycles in the hundreds, it is even more critical that effective wear-leveling is used to make MLC an acceptable choice for industrial and commercial uses. For this paper, we measured NAND flash configured for SLC, or Single-Level Cell. Another common Wear-Leveling critical for MLC flash media

FIGURE 1

flash is placed into that will primarily define the product lifetime. What is Wear-Leveling? Wear-leveling is a process to ensure that an entire flash memory device or an array of devices is used in a uniform fashion in order to extend the overall lifetime of the flash. For a simplistic example of wear- leveling, let’s look at a data recorder with these characteristics: • Application: The device collects and stores the past 24 hours of field data by simply writing and rewriting the data to the same location on the flash. • File size of data to be recorded: 128KB • Erase block size (of the flash): 128KB • Flash memory endurance: 1,000 cycles With one spare erase block, the device is assumed to use one cycle per day each year: (1,000 cycles ÷ 365 days) * 1 spare erase blocks = 2.74 years In this example, it would take about 2.74 years to cycle that one erase sector 1,000 times. For the data recorder device to accommodate the write erase rules

of flash memory, it would have to complete an erase operation to start writing the next day’s set of data. To make the data recorder more robust - to ensure that it doesn’t lose a whole day’s worth of data - we can set aside a second erase block, and erase the first block only after the second set of data was recorded. The resulting side effect is the introduction of a simple wear-leveling scheme. With two spare erase blocks, the device is assumed one cycle every two days each year: (1,000 cycles ÷ 365 days) * 2 spare erase blocks = 5.48 years With these parameters, the period of time prior to cycling the flash to its lifetime has just been increased to almost 5.5 years! This simple example shows how distributing a fixed set of writes across more flash sectors can increase the period of time prior to cycling the flash to its specified limits. The following sections describe how to account for the important variables associated with wear-leveling techniques, and determine the expected lifetime of the flash in any application.

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