EXTENDING LIFE OF STORAGE PRODUCTS BEYOND THE NAND SPECIFICATION
Fortasa Storage Products Offer Significantly Longer Usable Life Than the NAND Memory Components

Unlike Hard Disk Drives which offer unlimited ability to modify and overwrite programmed data, Flash memory based products have a finite and limited endurance ratings. Due to inherent technological limitations, Flash media deteriorates substantially with frequent program/erase cycling. Different Flash process technologies, and design variations offer substantially different technical characteristics, making some NAND Flash components not suitable for target application.

Fortasa's Flash controllers with sophisticated NAND Flash management techniques such as error correction, wear leveling algorithm, write filtering, bad block management, etc. significantly increase the usable life of Flash storage beyond what is specified by NAND flash manufacturers.

ADVANCED FLASH MEMORY CONTROLLER TECHNOLOGY TO EXTEND LIFE OF FLASH MEMORY
Wear Leveling

Every NAND flash device is architected in granularity of multiple Erase blocks that have to be pre-erased prior to writing data to them. Depending on process technology and design architecture, these blocks are limited in the number of Program/Erase Cycles they can endure before the probability of errors rise to unacceptable (and uncorrectable) levels. To reduce the likelihood of early storage system failure, it is extremely important that all blocks within a flash chip are aged evenly. If one specific area of the NAND device gets written to more frequently, while another never gets addressed, the lifetime of the entire memory device gets impacted, even if the majority of device address space was hardly used.

And to make matters even more critical, device drivers for the storage systems under legacy Operating Systems were written to support conventional HDDs that do not have any endurance issues that plague Solid State Storage. These drivers tend to update data multiple times to the same address location causing extreme duress to the Flash media block in that specific address location. Once the block fails, the whole storage device fails as any further writes to the stressed address location would not provide a reliable read of the programmed data.

To overcome this limitation, an advanced flash management algorithm is implemented in the Flash controller to ensure that the usage of the entire memory space of the Flash drive is equalized. Flash controller algorithm keeps track of the usage count for every memory block to make sure that flash media is used evenly across all blocks, maximizing flash lifetime. Once some of the memory blocks show greater usage than others, Flash controller automatically moves the data from least used blocks to the most used ones freeing up less used blocks to heavier write/erase cycling. In addition Fortasa's Flash controller reserves additional spare blocks to transparently replace the ones that had failed due to endurance stress. These advanced controller capabilities provide extended longevity to the Fortasa's storage solutions.

Error Correction/Detection

The Flash Controller inherent Error Correction algorithm has a direct influence on the Life Expectancy of the storage system. When a certain flash block approaches its endurance cycle limit, as specified by Flash memory manufacturer, it will exhibit a significantly higher probability of errors. A powerful and robust ECC mechanism can detect and correct these errors far beyond the Flash requirement extending the endurance of each erase block and, correspondingly, of the whole storage system.

Bad Block Management

NAND Flash inherently contains bad blocks. In addition, during run time, new bad blocks develop from accessing the data in the SSD. A sophisticated Flash memory controller maps out bad blocks as they occur to prevent catastrophic drive failure. Upon first time intialization, Flash controller allocates blocks into a usable and spare pool. During run time if one of the used blocks fails, the controller replaces it with a fresh spare block continuing drive operation transparent to the user. Fortasa storage products offer a SMART self-monitoring tool that can report Flash media usage, spare and failed block count and correctable errors, etc. to provide the drive "health" status to the system. These advanced Flash Controller media management features further increase the expected SSD lifespan.

Single Level Cell Flash (SLC) vs. Multi-Level Cell Flash (MLC)

Flash manufacturers specify a practical limit on how many write/erase cycles can be performed on a given erase block before the signs of wear-out become obvious and the probability of errors rise to unacceptable levels. Today's generation of Single Level Cell NAND Flash devices are guaranteed for 50,000 Program/Erase cycles per Erase Block with 1-bit ECC. Multi Level Cell NAND flash devices, while less expensive, are offered with only 3,000 cycles. The future generation of NAND Flash products will have an even more reduced endurance specification, curtailing the life expectancy of storage solutions. Depending on the critical requirements for customer end application, Fortasa offers appropriate storage solution balancing the application reliability and performance requirement with a cost effective storage choice.

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