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Part 6: The RAM SSD and NAND

The RAM Solid State Device:  The NAND SSD Forerunner

  1. Tape Drives
  2. Magnetic Drum Memory
  3. The Birth of the Hard Drive
  4. The 5.25-inch Hard Drive
  5. Limitations of the HDD
  6. The RAM SSD & NAND
  7. NAND in SSDs

In 1978, StorageTek introduced the first modern SSD.  This pioneering SSD had a maximum storage capacity of 90MB and sold for about $8,800 per megabyte. [42] "The SSD served the mainframe industry as a virtual memory extension for paging and swapping programs in and out of memory" [43]. That same year, Texas Memory Systems began marketing a 16KB RAM SSD to oil companies for a seismic data acquisition system. [44] SSDs were born, but didn’t take off. At least not right away.

As far as mainframes were concerned, "the arrival of expanded storage, a bus extension for additional main memory capacity, signaled the end of the SSD market—for a while," explained Fred Moore, a one-time StorageTek director. [45]

"In the early 1990s, a few small companies were building SSDs for select applications running on Unix, but market visibility was low and price per megabyte was still high. During the 1990s,  the popularity of Unix, NT, the Internet, and, later, Linux increased. They became the largest storage markets for databases, and the heavy I/O loads they generated created response time bottlenecks. Twenty-five years after their first appearance, SSDs are still a niche market but are becoming the new stealth weapon for system programmers and storage administrators who struggle to deliver the consistent response times necessary to meet service levels," [46] Moore wrote in 2002.

"Based on high-density DRAM chips, rather than rotating disk media and moving heads, the variable and lengthy seek and rotational times for rotating disks are eliminated, leaving a very short access and data transfer time to complete an I/O operation. There are no cache misses or back-end data transfers on an SSD. Typical I/O operations on an SSD occur between 30 and 40 times faster than on a rotating disk. SSDs are a quick fix for severe I/O performance problems, and they don't face the ongoing access density challenges of higher-capacity disks. These devices are fault-tolerant architectures and protect data from all types of device failures, not just from the loss of electrical power." [47]

In terms of a storage evolution, the DRAM- or RAM-based SSD was almost too specialized to have a large impact.

NAND Flash Technology

Fujio Masuoka began working on Flash memory cells in the 1970s at Toshiba and received patents for his work in 1980. [48] Masuoka's designs were perhaps the most important semiconductor innovation in the history of storage, but unfortunately, it went poorly for Masuoka. For his work Toshiba gave Masouka "a bonus worth a few hundred dollars"—and promptly let its archrival Intel take control of the market for his invention. Subsequently, Masuoka says, Toshiba tried repeatedly to move him from his senior post to a position where he could do no further research." [49]

Masouka's Flash memory concepts have evolved, and today NAND Flash technology and SSDs have the potential to displace HDDs and force an evolutionary step in storage.

Like all semiconductor devices, NAND Flash memory relies on an electrical current to operate. Specifically, a voltage "is applied to the control gate to draw electrons from the substrate to tunnel through the gate oxide into a polysilicon floating gate layer. To store one bit, two charge levels in the floating gate layer can be stored to distinguish between a 1 and a 0." [50]

Figure 9: NAND Flash Cell Programming

"Single-level cell (SLC) NAND Flash memory stores one bit of information per memory cell. This basic technology enables faster transfer speeds, lower power consumption, and increased endurance. For designs using mid-range densities, SLC NAND Flash will continue to be a good choice. Multiple-level cell (MLC) NAND, by comparison, stores two to four bits of information per memory cell, effectively doubling the amount of data that can be stored in a similar-size NAND Flash device. SLC NAND offers high performance and reliability, is supported by all controllers, and requires only 1-bit error correction code (ECC). SLC NAND is for applications like high-performance media cards, hybrid disk drives, solid state drives, and other embedded applications with processors, where it is used for code execution. MLC is supported only by controllers that include 4-bit or more ECC." [51]

MLC is a low-cost file storage solution for consumer applications like media players, cell phones, and media cards (USB, SD/MMC, and CF cards) where density is more important than performance. MLC NAND has also emerged as the dominant Flash memory choice for SSDs targeted at the notebook PC market because they offer such a well balanced price-to-performance solution.

Figure 10: Multilevel Cell Storage in NAND Flash

In fact, it is MLC NAND—for the most part—that has powered so many of the recent advances in mobile computing and digital media convergence. MLC NAND has replaced the day planner with the BlackBerry, exchanged film for media cards in cameras, and enabled a musical revolution with the Apple® iPod® and other MP3s. Today, people can carry more memory around in a USB drive on their key chains than an entire room full of early HDDs could have stored.

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Notes: [42] Fred Moore, "Enterprise Storage Report for the 1990s," StorageTek Corp., downloaded from http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930003944_1993003944.pdf. [43] Fred Moore, "What Goes Around Comes Around in Storage: Old Ideas Find New Applications for Today's SANs," Computer Technology Review, (September 2002), downloaded from http://findarticles.com/p/articles/mi_m0BRZ/is_/ai_101679012. [44] Gregory Wong, "Solid State Drives: A Closer Look Report No. FI-NHL-SSD-1008," Forward Insights, (October 2008): page 73. [45] Fred Moore, "What Goes Around Comes Around in Storage: Old Ideas Find New Applications for Today's SANs." [46] Fred Moore, "What Goes Around Comes Around in Storage: Old Ideas Find New Applications for Today's SANs." [47] Fred Moore, "What Goes Around Comes Around in Storage: Old Ideas Find New Applications for Today's SANs." [48] Benjamin Fulford, "Unsung Hero," Forbes (June 24, 2002), downloaded from http://www.forbes.com/global/2002/0624/030.html. [49] Fulford. [50] Wong, page 13. [51] "MLC vs. SLC Flash," downloaded from http://www.micron.com/nandcom/.

About Our Blogger

Dean Klein

Dean Klein is Vice President of Memory System Development at Micron Technology. Mr. Klein joined Micron in January 1999, after having held several leadership positions at Micron Electronics, Inc., including Executive Vice President of Product Development and Chief Technical Officer. He also co-founded and served as President of PC Tech, Inc., previously a wholly-owned subsidiary of Micron Electronics, Inc., from its inception in 1984. Mr. Klein’s current responsibilities as Vice President of Memory System Development focus on developing memory technologies and capabilities.

Mr. Klein earned a Bachelor of Science degree in electrical engineering and a Master of Electrical Engineering from the University of Minnesota, and he holds over 220 patents in the areas of computer architecture and electrical engineering. He has a passion for math and science education and is a mentor to the FIRST Robotics team (www.USFIRST.org) in the Meridian, Idaho school district.

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