Both Intel and Micron are selling solid-state drives (SSDs) based on their most advanced Flash parts and so both companies have internal incentives to cut per-bit Flash costs as quickly as they can both to make their NAND Flash ICs competitive and to drive down the cost of their SSDs. Cost is now the biggest obstacle for SSDs in their quest to become mainstream storage devices.

Also according to this article, Micron Technology posted its first profit in 3 years during the period ending December 3, 2009 with sales worth $1.74 billion. The company’s NAND business rose 21% quarter over quarter and its DRAM business rose by as much as 50% during the same period. Good tidings indeed for Micron and the Flash business.

The question’s not as simple as it seems. There’s a temporal quality to the question. What’s right ten years ago isn’t right today and probably won’t be right ten years from now. Semiconductor technology is both fluid and extremely dynamic. One thing’s certain. You need to deal with today’s problems today. If you can address the same problem in the same way two or three years from now, that’s great! But you still need to address today’s problem today. You need to use components you can get today, not some time in the future. The future may include some surprises that change today’s answer, but today’s answer must be based on what you can do today.

Why the emphasis on today? Well, any RAID server memory used today must be based on some sort of memory technology (or technologies) that’s commercially viable now. Researchers are working on more than a dozen new memory technologies that may someday produce a more ideal memory than the semiconductor memories we have at our fingertips today. It’s not clear when that might happen. Tantalizing technology announcements are made almost weekly. But technology announcements are generally light years away from being commercially competitive products and that’s never truer than when you’re talking about digital memory.

Bulletproof RAID server memory must have some mechanism to ride through power outages without data loss.  The previous two entries in this series (Part 1 and Part 2) discussed various approaches to creating bulletproof memory using battery-backed RAM. Seems like a great idea, but batteries aren’t particularly reliable in data-center environments where they live inside of heat-generating boxes squeezed into rack upon rack upon rack where they get no light and precious little maintenance. High-maintenance components like batteries just seem like a poor choice for creating memory that’s supposed to be bulletproof. Wouldn’t you agree?

So what’s that leave?

Well, you could use NAND Flash for memory rather than DRAM. NAND Flash devices have many excellent attributes. They do not require power to provide nonvolatile storage. They are currently the semiconductor industry’s cost-per-bit leader. NAND Flash chips available in higher capacities than DRAMs, which translates into more bits per same-size board, fewer devices per board for same-size capacity, or smaller boards depending on application needs. These are all great attributes.

Unfortunately, NAND Flash devices have some unhappy qualities as well. You can only write to them relatively slowly—much more slowly than DRAM. They also exhibit wearout failure, which is getting to be a bigger and bigger problem as lithographies shrink. NAND Flash devices are block oriented so you can’t write just one word. These three failings are major and make NAND Flash memories unsuitable for RAID server memories.

Unsuitable, that is, when used alone.

However, volatile DRAM paired with non-volatile NAND Flash make a pretty good team when it comes to building bulletproof RAID server memory. When the power’s good, use the DRAM like…well…DRAM. When there’s an indication that power’s about to fail, save the contents of the DRAM in NAND Flash devices.

Note that you can’t let the host CPU save the data when power’s already on the slippery downhill slope. You really don’t know how much time there is before the host CPU loses its mind. You need something more—bulletproof. You need a backup power supply that will sustain the memory subsystem during the data-backup operation and you need a local processor to oversee the transfer.

Batteries are still bad

The previous two installments of this series have already dealt with the many reasons that batteries are not suitable as the backup power supply. Barring the sudden invention of the Mr. Fusion portable reactor last seen attached to the back of Doc Brown’s DeLorean time machine in the Back to the Future movies, there’s really only one good alternative for emergency backup power for RAID server memories: ultra-capacitors.

Ultra-capacitors are capacitors that have electrodes with greatly expanded area, which result in greatly expanded capacitance. The electrode area expansion originates in porous carbon electrodes. Ultra-capacitors have capacities measured in Farads, much greater then conventional electrolytic capacitors. Although they require the proper care when designed into a backup power supply, ultra-capacitors can provide enough backup energy to support the emergency transfer of data from DRAM to NAND Flash memory in a bulletproof RAID server memory subsystem.

How practical is all this? Very practical. Take a look at the following graph, which plots projected memory costs in dollars per megabyte over the next few years. (This graph is based on iSuppli projections.)

As you can see, DRAM and NAND Flash are the least expensive semiconductor memories, per megabyte, and a megabyte of NAND Flash costs about one tenth of what a megabyte of DRAM costs. All of the leading “future” memories, which may someday replace DRAM, cost more. Some cost much more and they will continue to cost more into the foreseeable future. These “future” memory technologies are not about to replace DRAM today or tomorrow. They cost too much.

Finally note the dashed blue line. This line represents the per-bit cost of AGIGARAM, which fuses DRAM, NAND Flash, and ultra-capacitors to create the closest thing to a bulletproof RAID server memory that you can get today. Over time, the cost of a megabyte of AGIGARAM approaches the cost of the equivalent amounts of DRAM and NAND Flash added together. The cost of the memories will essentially dominate the other costs (controller, ultra-capacitor backup power source). Consequently, AGIGARAM, which is AgigA Tech’s bulletproof memory for RAID servers that’s available today, is not only the best technical approach to creating bulletproof memory, it’s the most cost-effective approach available today…and tomorrow.