The hows and whys of SSDs
Solid State Disks are poised blow the doors off of traditional storage media. As the inevitable end-game of the great bet on flash memory, they are coming in strengthening numbers to obliterate benchmarks, make or break companies, and free-fall in price. The revolution this nascent market is set to unleash will leave few questions as it makes a staggering rise to preeminence.
The history of flash
Flash began humbly in the laboratory of Dr. Fujio Masuoka as Toshiba sought to address the need for inexpensive non-volatile memory that could be easily reprogrammed. During early testing of the fledgling technology, a colleague by the name of Shoji Ariizumi commented that erasing data by using a sudden flash of electricity was similar to the flash of a camera. While the memory being introduced to the 1984 IEEE Electron Devices Meeting was known as “NOR,” Ariizumi’s unwitting comment has become the household name.
Intel was also present at the 1984 IEDM and was quick to recognize the potential of NOR-type flash. The Santa Clara-based foundry was first to commercialize the visionary technology in 1988 with the release of the 256Kb IC which, at $20 USD, commanded the arresting sum of $640 USD per megabyte. In spite of the numbing price, the introduction of commercial NOR memory was a sensational success that spawned numerous companies in its wake. It was clear that Toshiba’s work had triggered something much larger than originally anticipated.
At 1989’s International Solid-State Circuits Conference, Dr. Fujio Masuoka and Toshiba once more excited onlookers with the introduction of NAND-type flash memory. NAND surpassed NOR with enhanced longevity, faster I/O, lower costs and a smaller footprint amongst its features. While NOR was first to market in commercial devices with SanDisk’s 1994 CompactFlash I devices, NAND came to stay just a year later with Toshiba’s 1995 release of the SmartMedia specification.
Though NAND remains the flash memory magnate, considerable research continues for both types of flash memory. Manufacturers hope that their research will meet the capacious demands of users while continuing to enhance the speed, cost, and reliability of flash-based devices.
How flash memory works
Flash memory’s radical approach to storage technology dispenses with mechanical components and represents information with the electron. To harness those electrons, solid state disks begin with the flash memory cell.
Each cell is composed of nine major components: the word line, bit line, control gate, floating gate, oxide layer, drain, source, ground, and substrate. These unbelievably tiny cells — millions of which are arranged in an electrically-connected grid — serve as the building block of today’s flash devices.
The structure of NAND flash cell. The black lines represent current paths with or without wires.
All solid-state memory is designed to record states, or the strength of an electric current which represents a binary digit. Technologies like RAM lose their programmed information when the current is severed as they have no method to retain the electrons that represented information. Conversely, NAND can preserve its states by trapping the electrons with a process known as Fowler-Nordheim Tunneling.
The process begins by applying a positive current of approximately twelve volts to the word line and bit line. The positive charge on the bit line pulls a rush of electrons from the source to the drain as the current flows to ground. On the word line, the charge is sufficiently strong to tug a few electrons away from their race to the drain. While the oxide layers are typically a powerful insulator, the excited electrons are able to surmount this barrier and become trapped within the floating gate. These trapped electrons are how flash memory is able to remember the electrons that represent information.
The induction of an electric field (blue outline) along the lines excites electrons and forces them through the oxide layer to become trapped in the floating gate.
Reading information back out of a flash cell is done by a sensor that compares the charge of the trapped electrons against a steady current. If the charge in the gate exceeds fifty percent of the current’s strength, the cell is considered to be “closed” and represents a zero. If the current can move through the floating gate without being impeded by captured electrons, the gate is considered “open” and represents a binary one.
Flash memory employs blocks composed of thousands of NAND cells. Each block uses a common word and bit line.
Exceptional article as always, Thrax. Excellent coverage and breadth.
The width and girth of this treatise are exorbitant!
These drives can really help out laptop performance IMO. Laptops have those really slow rotational speeds (usually 5400RPM) which cuts into performance more than you would think, especially high end ones.
SSD have a long way to go before I'll even consider one.
Recently, I started seriously looking at getting a solid state drive (SSD) as my primary boot drive. After careful consideration, I have concluded that they still are not ready for prime time from the enthusiast gamer's point of view. The two biggest deterrent factors are the cost of SSD's and their life expectancy. As of today, an Intel X25-M SATA Solid-State Drive costs $US595 in quantities of 1000. Another very disturbing issue is the fact that regular defragmentation of a solid state drive would dramatically decrease it's life expectancy. As it stands, the earliest I see myself having an SSD is sometime around 2010.
Fragmentation is not an issue. SSDs intentionally fragment files across the drive in a process called "wear leveling." Wear leveling assures that no one flash cell gets more work than others, thereby extending the life of the drive. If a file were stored in 100,000 places or in one contiguous block, an SSD would be able to load that file at the same speed.
Defragmentation is a cheap hack to sweep the performance limitations of mechanical drives under the rug. Defragging exists because there are performance penalties if the mechanical drive head needs to see files all over the disk.
Secondly, the longevity (MTBF) of the newest generation of Intel SSDs is as long or longer than traditional drives. Reliability has reached parity, it's not really a concern any more.
I do, however, agree that the price needs to come down.