Choosing the best NAND controller for the job
NAND flash storage has evolved steadily over the last decade, to the point where it has replaced the HDD as the most widely used mass storage medium. However, NAND flash on its own is only part of the story, since no solution would operate without a NAND flash controller, the brain behind any NAND flash storage system.
Controllers differ widely in their range of functions and features, and the storage system designer must choose carefully to ensure the optimum trade-off between performance and reliability as well as cost and benefit. In this blog post we look closely at the role of the NAND flash controller and discuss some of the characteristics and features that should be considered when making this decision.
The Ubiquitous NAND Flash
NAND flash’s popularity as a storage medium is based on its low power consumption, along with its high speed and high reliability. The cost per byte of the technology has been driven down by a succession of techniques designed to increase storage density, with modern flash memories able to store two, (MLC), three, (TLC) of four, (QLC) bits per cell. 3D TLC has become a popular choice for NAND flash, since it offers a good balance between cost reduction per byte and storage life and performance.
Despite its many benefits, NAND flash is inherently unreliable as memory cells deteriorate over time due to electron migration, which is accelerated by the heat generated during operation. An aging cell loses its charge state, leading to a “bit flip” error in the stored data and semiconductor devices with smaller cell structures are more susceptible to these aging effects. Detection and compensation of these errors is one of the many functions of the memory controller, an essential component of any NAND flash storage system.
The Memory Controller, an integral part of any NAND Flash Storage System
The memory controller ensures the integrity of data passing between the flash memory and the host. Read/write/status commands from the host are translated into a format which can be understood by the NAND flash. The controller also translates the logical block addresses, (LABs) of the host’s file system into addresses on the flash memory while managing the flash deficiencies discussed above.
These devices vary significantly in cost and sophistication, with the most suitable for any given storage system being dependent on the needs of the application. Controllers are generally either DRAM-based or DRAM-less, figure 1, with the latter being most suited to high-reliability, industrial applications such as medical devices or mobile base stations. Although DRAM-based controllers may offer higher performance, they can be vulnerable to power failures where data stored in the DRAM caches can be lost.
Controllers also differ in the way in which they deal with bit errors in the NAND flash. While most include a refresh algorithm to detect data old and unstable data, less expensive devices only perform checks when the host requests a read operation. More sophisticated controllers include scheduled drive scans within routine background maintenance operations.
Self-Monitoring, Analysis and Reporting Technology (SMART) is a feature which provides health information of the NAND flash memory, enabling the user to monitor the life of the flash device based on various attributes. Memory lifetime is estimated based on number of spare blocks, erase operations, total number of reads or total number of ECC errors. Standard on ATA interfaces, hySMART™ is also available by Hyperstone on a range of their controllers.
Choosing the right controller for the application
Choosing the right flash memory controller for a storage solution is key to achieving the optimum balance between performance and reliability as well as cost and benefit. The reliability needs of a consumer product operating for a six-hour day over a life cycle of 5 years will be very different from those of an industrial product operating for over ten years in a hostile environment. With many options on the market, insight and experience is required to inform this choice and, with over 30 years’ experience in the production of NAND flash memory controllers, Hyperstone is well-placed to help its customers make this choice.
Whether it be an advanced device such as the new S9 SD controller, designed to meet the needs of the most demanding applications or through Hyperstone’s extensive design and consulting support, there is something in Hyperstone’s portfolio to ensure that the right choice is made.