Each new generation of NAND flash memory reduces the fabrication geometry – the dimension of the smallest part of an integrated circuit used to build up the components inside the chip. That means there are fewer electrons storing the data, leading to increased errors and a shorter life for the flash memory. No need to worry. Today’s flash memory depends upon the intelligence and capabilities of the solid state drive (SSD) controller to help keep errors in check and get the longest life possible from flash memory, making it usable in compute environments like laptop computers and enterprise datacenters.
Today’s volume NAND flash memory uses a 20nm and 19nm manufacturing process, but the next generation will be in the 16nm range. Some experts speculate that today’s controllers will struggle to work with this next generation of flash memory to support the high number of write cycles required in datacenters. Also, the current multi-level cell (MLC) flash memory is transitioning to triple-level cell (TLC), which has an even shorter life expectancy and higher error rates.
Can sub-20nm flash survive in the datacenter?
Yes, but it will take a flash memory controller with smarts the industry has never seen before. How intelligent? Sub-20nm flash will need to stretch the life of the flash memory beyond the flash manufacturer’s specifications and correct far more errors than ever before, while still maintaining high throughput and very low latency. And to protect against periodic error correction algorithm failures, the flash will need some kind of redundancy (backup) of the data inside the SSD itself.
When will such a controller materialize?
LSI this week introduced the third generation of its flagship SSD and flash memory controller, called the SandForce SF3700. The controller is newly engineered and architected to solve the lifespan, performance, and reliability challenges of deploying sub-20nm flash memory in today’s performance-hungry enterprise datacenters. The SandForce SF3700 also enables longer periods between battery recharges for power-sipping client laptop and ultrabook systems. It all happens in a single ASIC package. The SandForce SF3700 is the first SSD controller to include both PCIe and SATA host interfaces natively in one chip to give customers of SSD manufacturers an easy migration path as more of them move to the faster PCIe host interface.
How does the SandForce SF3700 controller make sub-20nm flash excel in the datacenter?
Our new controller builds on the award-winning capabilities of the current SandForce SSD and flash controllers. We’ve refined our DuraWrite™ data reduction technology to streamline the way it picks blocks, collects garbage and reduces the write count. You’ll like the result: longer flash endurance and higher read and write speeds.
The SandForce SF3700 includes SHIELD™ error correction, which applies LDPC and DSP technology in unique ways to correct the higher error rates from the new generations of flash memory. SHIELD technology uses a multi-level error correction schema to optimize the time to get the correct data. Also, with its exclusive Adaptive Code Rate feature, SHIELD leverages DuraWrite technology’s ability to span internal NAND flash boundaries between the user data space and the flash manufacturer’s dedicated ECC field. Other controllers only use one size of ECC code rate for flash memory – the one largest size designed to support the end of the flash’s life. Early in the flash life, a much smaller size ECC is required, and SHIELD technology scales down the ECC accordingly, diverting the remaining free space as additional over provisioning. SHIELD partially increases the ECC size over time as the flash ages to correct the increasing failures, but does not use the largest ECC size until the flash is nearly at the end of its life.
Why is this good? Greater over provisioning over the life of the SSD improves performance and increases the endurance. SHIELD also allows the ECC field to grow even larger after it reaches its specified end of life. The big takeaway: All of these SHIELD capabilities increase flash write endurance many times beyond the manufacturer’s specification. In fact at the 2013 Flash Memory Summit exposition in Santa Clara, CA, SHIELD was shown to extend the endurance of a particular Micron NAND flash by nearly six times.
That’s not all. The SandForce SF3700 controller’s RAISE™ data reliability feature now offers stronger protection, including full die failure and more options for protecting data on SSDs with low (e.g., 32GB & 64GB) and binary (256GB vs. 240GB) capacities.
So what about end user systems?
The beauty of all SandForce flash and SSD controllers is its onboard firmware, which takes the one common hardware component – the ASIC – and adapts it to the user’s storage environment. For example, in client applications the firmware helps the controller preserve SSD power to enable users of laptop and ultrabook systems to remain unplugged longer between battery recharges. In contrast, enterprise environments require the highest possible performance and lowest latency. This higher performance draws more power, a tradeoff the enterprise is willing to make for the fastest time-to-data. The firmware makes other similar tradeoffs based on which storage environment it is serving.
Although most people consider the enterprise and client storage needs are very diverse, we think the new SandForce SF3700 Flash and SSD controller showcases the perfect balance of power and performance that any user hanging ten can appreciate.
For the uninitiated, low-density parity-check (LDPC) code is an error correction code (ECC) that is used to both detect and correct errors on data that is transmitted from one point to another. All ECC types include correction data, so when information is transmitted with errors, the receiver has enough information to fix the errors without having to ask the source for the data again.
This enables transmitted data to maintain a constant speed as is required with digital television signals. What you don’t want is for the image to freeze repeatedly while waiting for correction data to be sent multiple times.
LDPC code was first presented to the world by Robert G. Gallager at MIT in 1960. It was very advanced for its time and, as it turned out, required a fantastic amount of computation to use real-time. The problem was that back in 1960, vacuum tube computers of that period performed about 100 times less work than the microprocessor-powered computers of today. In 1960, you would need a computer the size of a 2,000-square-foot house to process the LDPC correction information in real-time. This was hardly economical, so LDPC was mostly lost for nearly 40 years, as other simpler codes took its place over that period.
What was old is new again
In the mid-1990s, engineers working on satellite transmissions for digital television dusted off the LDPC codes and started using them for real-time operations. By then, computer processing had seen dramatic reductions in size and costs. Fast-forward to the past 5 years: we have seen a major increase in LDPC development and use because it appears to be the best solution for high-speed data transmissions, especially those subject to heavy levels of electrical noise that induce higher error rates. Also, the processing power of target devices like WiFi receivers and HDDs has grown even stronger and faster than some of the main CPUs of a few years ago. This enables LDPC to be deployed for little additional cost with the advantage of real-time data correction superior to correction offered by simpler codes.
If you have seen one LDPC solution, have you seen them all?
Nothing could be further from the truth. For example, an LDPC solution designed for satellite communication cannot be used for HDDs since there is no direct porting of the code, though there are distinct advantages to the two engineering teams sharing their knowledge and experience through their development efforts. Take, for example, the LDPC code that LSI has been shipping in its TrueStore® HDD read channel solutions for 3 years now. When LSI acquired SandForce and started work on SHIELD™ error correction code (based on LDPC) for flash controllers, there was no direct porting of that HDD code to support SSDs. However, the HDD development team’s knowledge and experience from creating the HDD code greatly improved the SSD team’s ability to more quickly bring SHIELD technology to the next-generation SandForce flash controller.
How do LDPC solutions for SSDs differ?
Many LDPC providers claim that their offerings rival the capabilities of competitive solutions, though often they aren’t telling the whole story. All LDPC solutions start with what is called hard-decision LDPC – a digital correction algorithm that operates at line rate on all data passing through the correction engine. The algorithm uses the meta-data generated from the user and system data stored on the flash memory, and helps recreate the user data when the flash memory returns it with errors. Hard-decision LDPC catches most errors from the flash memory, though sometimes it can be overwhelmed by an inordinate number of errors. That is where soft-decision LDPC – a more analog-based correction algorithm – comes into play.
Can a soft-decision be strong enough for my data?
Soft-decision LDPC is an error correction method that looks at other information beyond the actual ECC data. Soft-decision, in a sense, looks at the meta-data of the meta-data. The simplest form of soft-decision LDPC may just re-read the data at a different reference voltage, as if asking a person “can you say that again?” More complex soft-decision might be compared to listening to a man with a heavy French accent speaking English. You know he just said something in English, but you could not clearly grasp what he said. You ask some questions and, from his answers, soon realize what he originally said and are now back on track. While this might seem more like guessing at the answer, soft-decision LDPC uses statistics to help ensure the answers are not false positive results. As a result, soft-decision LDPC uncovers a new set of engineering problems that need to be solved, opening new opportunities for flash controller manufacturers to create powerful intellectual property (IP). For that reason, you’re likely to learn very little about how a given company’s soft-decision LDPC works.
At the 2013 Flash Memory Summit in Santa Clara, California, LSI demonstrated its SHIELD Advanced Error Correction Technology. SHIELD technology includes hard and soft-decision LDPC with digital signal processing (DSP) and a number of other unique features designed to optimize future NAND flash memory operation in compute environments. One feature, called Adaptive Code Rate, works with other LSI features to enable the spare area in flash memory reserved for ECC data to occupy less space than the manufacturer allocation and then dynamically grow to accommodate inevitable increases in flash error rates. The soft-decision LDPC capability offers multiple strengths of correction, with each activating only as necessary to ensure the lowest possible real-time latency.
So it’s clear that all LDPC solutions are far from the same. When evaluating LDPC solutions, be sure to understand how they manage data correction when it exceeds the ability of the hard-decision LDPC. Also, make sure the algorithms are actually in use in a product. Otherwise, the product might turn out to be a science experiment that never works.
Tags: Adaptive Code Rate, ECC, error correction, flash controllers, flash memory, Flash Memory Summit, LDPC, NAND flash, Robert Gallager, SandForce, SHIELD, solid state drive, SSD, TrueStore