Solid State Disk (SSD) & Flashcards

Most users are familiar with flash memory from its use in consumer electronics products such as digital cameras, MP3 players and USB memory sticks. Manufacturers have now combined flash memory with controller technology to produce the solid state drive (SSD). Solid state drives interface with their host system in the same way as a disk drive but data is stored and accessed within flash memory as opposed the spinning platters. As technology has advanced manufacturers are now able to produce designs that begin to meet the capacity, performance and   power requirements of server and storage environments.

Flash Memory Technology  

Solid state drives are based on flash memory technology; this is a non-volatile memory that can be electrically erased and reprogrammed. Being non-volatile means that no power is required to maintain the data stored in the chip which give SSD a 50% overall improvement on power consumption & cooling over standard HDD technology. In addition to this flash memory offers faster access times & read/write performance making it ideal for I/O intensive programs and also better kinetic shock resistance than hard drives which reduces failure rates and data loss. Other benefits include reduced noise and extended temperature operating ranges making SSD ideal for certain embedded applications.

There are two types of flash memory commonly referred to as NAND flash and NOR flash. Both NAND and NOR flash store information in arrays of cells but differ in the way the cell arrays are organised and accessed. In NOR flash, cells are connected in parallel to the bit lines allowing cells to be read and programmed individually. In NAND flash, cells are connected in series and therefore can only be read or programmed in series as a group.

The design of NAND memory architecture mean that NAND memory arrays can be created with almost twice the density of comparable NOR memory, and at a lower cost. This has resulted in NAND flash being the predominate architecture in the marketplace and will remain so in the immediate future.

Single-level (SLC) and multi-level cell NAND flash

There are two types of NAND flash technology: single-level cell (SLC) and multi-level cell (MLC). Single-level cell technology works by holding a single level of charge in each cell which represents a single bit of information. Multi-level cell technology stores one of four different charge states in a cell. This allows each cell to represent 2 bits of information, effectively doubling the memory storage over SLC flash memory.

MLC technology has fast become the predominant flash technology used in the broader markets for consumer products due to its higher capacity and low price point. However compared to SLC, MLC has characteristics that make it less appealing to higher performance; higher reliability devices used in server and storage environments such as:

• Higher internal error rates
• Significantly shorter lifespan in terms of number of program/erase cycles
• Slower read performance and significantly slower write (program) performance

Summary

Continuing developments in NAND flash and SSD controller technologies are leading to rapid advances in performance, reliability and storage capacities for SSD’s. The emerging devices represent a new product class for server and storage applications that we will see growing in the coming years. In 2008 word wide SSD revenues were estimated at $77 million and these are expected to grow quickly with IDC forecasting the market to be worth $1.2 billion by 2011, this represents a 76% growth rate.

There are several drivers that are fuelling OEM and end user adoption of SSD’s. In addition to the obvious performance advantages over HDD, declining prices and demand for energy conservation has driven early adoption. Since SSD’s have no mechanical parts, energy and cooling costs drop significantly, making SSD technology a natural for “green” environments