Solid State Drive (SSD)

White Paper

July-03-2009

As the computing power of the PC increases, the one bottleneck remains to be the hard disk. The hard disk is one of the few components of the PC that is still relies on mechanical gears and motors to shuffle data back and forth. The mechanical makeup of the hard disk makes it difficult to transfer data efficiently. Even with advance design in high speed micro motors and optimal data placement, seek time is still unavoidable. Another undesirable factor common with the hard disk is heat. With all the mechanical components feverishly working during data reads and writes, a great deal of heat is generated as a byproduct.

Solid State Drive (SSD) diagramSolid State Drive, also known as SSD, is a type of storage which is based on flash memory devices. One common type of flash used in SSD is NAND flash. One of the advantages of using flash to store data is that the seek time is virtually non existent. Data retrieval is almost instantaneous thus reducing seek time significantly. Another advantage is the heat generating from the flash and the logics are very low. This makes SSD an attractive option if the computing environment is in extreme conditions. Because there are no moving parts on the SSD it less vulnerable to damage cause by shock and vibration. Let’s look more in details the structure of the NAND flash.

NAND flash is an array of transistors in series which are known as cells. A cell is comprise of two gates separated by a thin dielectric oxide layer. One gate is called the floating gate and the other is the control gate. An insulating dielectric surrounds the floating gate. When there are no excess of electrons on the floating gate, the cell is in the erase state or logic “1” and has a low threshold
voltage. When the cell is in the programmed state or logic “0”, it has a high threshold voltage. There are 2 types of NAND, Single Level Cell (SLC) and Multi Level Cell (MLC). SLC NAND holds one bit per cell, whereas MLC holds two bits per cell. The advantages of SLC over MLC are higher program/erase cycles and faster programming time. Because of its higher program/erase cycle, SLC is more appropriate for use in frequently update clients.

With the program/erase cycle of 100,000 and 10,000 for SLC and MLC, respectively, using NAND as a storage device with high write frequency will render it useless only after a short period. A technique called wear-leveling is implemented in SSD to increase the life cycle of the NAND flash. Wear-leveling distributes the program/erase evenly throughout the flash in order to minimize the wear thus prolonging the flash life cycle.

As with hard disk storage, flash storage also contains bad blocks. SSD’s other feature is bad block management- blocks which do not program/erase correctly will be marked and will not be used. Because of bad block management, logical to physical addressing is required in order to store the data properly. SSD also has built-in ECC function for added data integrity.

In conclusion, SSD unique properties make it an attractive option to traditional rotating disk drives. Its low latency, low heat, high shock tolerance and compact size offer an ideal storage solution for extreme operating environments.