Kingston SSDNow V+ G2 | Accessories

Kingston SSDNow V+ G2 - Technical explanations

Friday, 05 February 2010 07:45 Laptopfreak

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Article Index
Kingston SSDNow V+ G2
Specifications
Technical explanations
Package contents
Benchmark
Conclusion
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Page 3 of 6

Technical explanations

What is SSD
SSD stands for solid state drive. It is based on NAND flash memory, of which there are two kinds: MLC (multi-level cell) and SLC (single-level cell).

(Click on the picture)

In the field of electronics, a multi-level cell (MLC) is a memory device capable of storing more than a single bit of information.

MLC NAND Flash is a flash-memory technology which – by using several levels per cell – allows more bits to be stored as opposed to SLC NAND flash-technology, which only uses a single level per cell.

At the moment, most MLC NAND units store four states per cell, so the four states give two bits of information per cell. This reduces the margin that separates these states, resulting in a larger error rate. MLC NAND has the advantage of being cheap due to the more compact storage method, but this requires greater software complexity to compensate for the higher error rate.
The higher error rate requires an algorithm that can correct errors up to five bits and identify states with more than five bad bits. The most commonly used algorithm is Bose-Chaudhuri-Hocquenghem (BCH-code), but let us not dwell on the specifics.

SLC units work the other way around, using two cells per bit to give an even lower rate of error.

SSDs contain more than memory chips, however; another important component is the controller. It “controls” all the data that goes back and forth, and is where the complicated algorithm required for MLC SSD must be carried out. In fact, MLC requires three times more raw processing power than SLC, so the controller is of vital importance to performance. The firmware that manufacturers employ can often optimise your drive considerably with a new and improved algorithm or the like, so it is advisable to look out for SSD firmware updates, although watch out for beta (test) versions.

What is TRIM command
Most operating systems utilise writing blocks of 4KB. If you want to overwrite 4KB of data from the full blocks on an SSD, 512KB of data must be deleted and rewritten. This problem is solved by TRIM.

When Windows shows, for example, there is only 100GB of capacity on the drive, there is more in reality. This is because the OS has designated an area to be used for moving data, and so it is hidden and unavailable to the user. This “buffer” corresponds to about 6-7% of the drives total capacity. This buffer is typically increased in size on SSDs to increase performance.

TRIM allows the SSD drive to prioritise which blocks to clean. The OS provides the drive with the address of the cells that are to be deleted, but lets the drive decide when the action is carried out. When data is deleted from a block, it moves the valid data to the buffer area and the block is marked for deletion. The drive then decides when exactly a give block is deleted and overwritten. Sort of like in Tetris, data is only overwritten when there is an entire block, i.e. 512KB.

You can compare it with the defragmentation of an ordinary hard drive – the difference is just that the SSD carries out the task, rather than the OS.

What is IO/s (IOPS)
An abbreviation for Input/Output Operations Per Second. This can be viewed as how many of either inputs or outputs of a given block size the drive can handle per second. Thus IOPS must be higher the smaller the blocks KB you want to read or write, given that you want to retain an average read/write speed at all block sizes. By extension, the lower the amount of IOPS required, the greater the block size in KB.

What is Access time
The time is takes the drive to access the segment it seeks.