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Adtron Flash Disks: Reliability Begins at the Flash Memory Level

Flash disks are widely regarded as the most reliable form of data storage media. To ensure reliability, Adtron characterizes the flash memory to verify the flash manufacturer's specifications before the media ever makes it into an Adtron flash disk. Each generation of flash memory introduced by a manufacturer is put through extensive testing and validation. Adtron uses the following process to arrive at a solid state disk storage system that seamlessly functions as a hard disk or tape drive:



The first step is to evaluate only the highest quality NAND flash available on the market today. Changes in flash die geometries, cell structure and address control functions impact yield rates from the wafer during the fabrication process. The concept that certain spots on a wafer are more likely to produce a higher quality chip is prevalent throughout the semiconductor industry. When flash memory wafers are tested and probed, a distribution of bad, weak, and strong die are identified across the wafer. Bad cells are marked, and the remaining cells are sorted into consumer and industrial quality flash. Adtron selects only industrial quality flash identified at the die sort stage. Consumer grade flash makes it into flash disks that are mass marketed through retail outlets.

After identifying the industrial-grade flash memory, Adtron continues the characterization process by independently testing and validating the manufacturer's specifications in our facilities in Phoenix Arizona. At this stage the flash memory is put through extensive write endurance testing and the disturb characteristics are evaluated to further ensure the reliability of an Adtron flash disk.

Write endurance testing involves continuously writing to flash cells by sending charges to the floating gate. Every write or erase operation that is performed erodes the layer that holds the data charge. Twenty years ago, when flash disk technology first became available, write endurance was only a few thousand cycles. Today, and with advances in technology and materials, flash cell write endurance now conservatively exceeds 2 million erase/write cycles. Through Adtron testing and advanced design, Adtron flash disks tested in our lab have exceeded well over 5,000,000 cycles. SSD write integrity far exceeds traditional magnetic disk integrity.

Disturb errors are encountered when operations performed on one flash block affect the integrity of other flash blocks in the vicinity. Using Error Correction Code (ECC) Algorithms to compare the ECC of the previous read/write operation with that of the most recent ECC calculation disturb events are detected and corrected. Flash disk disturb errors are corrected in one of two ways, by either rewriting the data to the existing flash block or to a different available flash block. All flash disk disturb error handling is resolved internally and undetected by the system.

Just like with traditional hard drives internal algorithmic techniques are used to extend the life of the flash disk including sector sparing. After it has been determined that the flash memory meets Adtron solid state disk requirements for write endurance and disturb error handling, the third phase of flash characterization involves conducting tests on the flash controller algorithms to validate sector sparing and wear leveling capabilities.

Sector sparing is the process in which a percentage of the flash capacity is reserved for new flash cells to replace failed cells and thereby erase/write endurance is extended. This also helps to avoid other issues and critical failures. Unlike traditional hard drives, flash disk sparing is not limited by a certain number of available tracks allowing flash devices to spare over a wider area and utilize more of the flash capacity and add extended life to the disk.

Wear leveling works in tandem with sector sparing to extend the life of a disk by spreading write activities evenly over logical sectors of the physical flash disk. By utilizing this technique and spreading wear and tear experienced during normal use across the entire disk the conditions that lead to disturb errors are avoided.

The final phase of characterization is to take the fully integrated flash technology that has become an Adtron flash disk and generate mean time between failure (MTBF) statistics and conduct highly accelerated life testing (HALT) tests. For more information on these topics, refer to Solid State Disk Reliability Predictions.

To learn more about Adtron and its flash disk products contact us today at 602.735.0300.