Know Your SSDs - SLC vs. MLC

选SSD，请认准SLC

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Memoright 128Gb SSD Review – Part 2 or

Know Your SSDs - SLC vs. MLC

By Les Tokar

Introduction

This article is written to follow in the footsteps of an excellent review by Kevin O’Brien on the Memoright 128Gb solid state drive (SSD). His article provided many great benchmarks, to include HDTune, HDTach and ATTO Disk Benchmarking, which have gained a reputation of accuracy and reliability over the past few years. It’s refreshing to see a secondary reviewer who can validate claims of the capabilities of SSD’s along with offering his own opinions, a positive characteristic we see in all reviews here at NBR.

In this article, we are going to build on the first by showing why the Memoright 128Gb SSD is so unique and will mostly likely hold a very solid footing in SSD sales for a long time to come. We will do this by explaining a bit of the inner componentry of the SSD, performance considerations, why they are priced as they are along with how this will soon change creating a flurry of sales in the consumer SSD market within the next year. 

This article is a MUST READ for anyone contemplating buying a SSD in the near future and will most likely hook many who are walking that line in the sand.

As a bonus, we are going to reveal information that is presently a very sensitive spot with many manufacturers. This information pertains to the surprising lifespan of soon to be manufactured SSDs that you will need to know and understand.

Every Little Bit Counts

The SSD, at its most basic level, is comprised of bits, lots and lots of bits. To try and build from the bit and explain how we reach a total of 128Gb SSD would scare most off. Understanding the bit, however, will bring us to an understanding of two different types of SSDs, the slc (single level cell), or the mlc (multi level cell) design. Our understanding of these two is crucial to our SSD purchase.

An slc SSD, such as the Memoright 128 GB SSD, is simply built where 1 bit rests within 1 cell of the SSD. Conversely, a mlc SSD means there are two bits within 1 cell of the SSD. The best way of understanding this is to compare a cell with a single bit to a single home whereas a cell with two bits would be a duplex. For those capable of using the slc SSD to its full potential, it is easily understood that they have much more control over the total SSD as each bit is within it’s own cell. Its much the same as trying to make alterations on your single family home, vice a duplex.

Where we need to pay attention, however, is with failure of the SSD as it occurs at the cell level. In a duplex, both sides are lost by fire vice only the one with a regular home. This is much the same as losing both bits in a mlc, vice the single bit of the slc. This directly affects the lifespan of the mlc compared to the slc which will be covered ‘in just a bit’.

Suffice it to say, we can form a basic understanding that the slc SSD, as with the Memoright 128Gb design, is much better than the mlc SSD.

How Fast is Fast?

Presently, the only SSD capable of reaching performance results in excess of 100MB/s read and write at the 128Gb capacity is the Memoright 128Gb SSD. This may remain as such because many consumer manufacturers seem to be experiencing a problem simply fitting 128Gb NAND into a 2.5” SATA SSD. Having spoken to several, they seem to agree on the simple fact that, in no time soon, can the slc design reach the average consumer at 128Gb or above, for the most part because it cannot be made at a cost effective price.

As a result, manufacturers are furiously testing and manufacturing mlc SSDs for consumer sales because they can reach higher capacities at a cost within the consumers reach. There are performance hits though. Most manufacturers admit that mlc SSDs will rest in the area estimated at a maximum of around 90MB/s read and 50MB/s write, this well below the slc design.Could this be good news for the Raptor? Read on!

The Performance Misconception 

The true performance of an SSD cannot be understood unless one has had the opportunity to utilize one. Even watching a short demo is not a fair representation. We are so used to grading the performance of a HD by its read and write benchmarks that we instinctively look to this from the SSD as well.

The magic of the SSD comes less so from the read and write benchmarks and more so from the incredible access speed that it can achieve. An SSD searches similarly to that of your computer. It finds something by index and then retrieves it from NAND as the computer would from RAM. 

There are no moving parts involved and this occurs as quickly as 0.1ms.

With the typical consumer owned hard drive, it takes 10-15ms, on average, to find information and retrieve it because of the disk spinning, arm pivoting above and having to find the information on each pass and so on. This is compounded by the fact that a large file on a HD can take several passes to retrieve, each pass being 10-15ms on average compared to the 0.1ms total time of the SSD. This is why so many programs seem to open instantly with an SSD vice the HD. This is the true guts of the SSD.

How Long Will They Last

I am very close with many SSD manufacturers of both the enterprise and consumer arena. I have asked this question of many and must admit that the most frank response is “MLC lifespan is a little sensitive with manufacturers. Each is trying to cope with this answer very carefully.” The true answer to the question for both slc and mlc SSDs is this…

“The SSD will outlive the hardware for which it was built for.”

SLC ssds can be calculated, for the most part, to live anywhere between 49 years and 149 years, on average, by the best estimates. The Memoright testing can validate the 128Gb SSD having a write endurance lifespan in excess of 200 years with an average write of 100Gb per day.

This is where the mlc design falls short. None have been released as of yet. Nobody has really examined what kind of life expectancy is assured with the mlc except that, it will be considerably lower. I have received several different beliefs which average out to a 10 to 1 lifespan in favour of the slc design. A conservative guess is that most lifespan estimates will come between 7 and 10 years, depending on the advancement of ‘wear leveling algorythms ’ within the controllers of each manufacturer. 

To draw comparison by way of write cycles, a slc would have a lifetime of 100,000 complete write cycles in comparison to the mlc which has a lifetime of 10,000 write cycles. This could increase significantly depending on the design of ‘wear leveling’ utilized.

Understanding Pricing

DVNation sells the Memoright 128Gb SSD for $3399. There is a lot of reasoning behind this, the first of which is the price of NAND. It presently floats around $11/Gb on an 8Gb chip (slc) which has dropped some 60-70 percent in the last year. To understand why mlc ssds are going to arrive much cheaper, we need to understand that mlc NAND sells for considerably less, at roughly $3.50 per Gb at the time of this writing some 63% cheaper. You can watch the price of NAND here at the bottom table entitled ‘Flash Contract Price’. The NAND which we are looking at is 8Gb 1024Mx8 (slc or mlc) and the price point reflects cost per gigabyte. Below, we can get an idea of the dropping price of NAND over the last two years.

From here we also need to look at manufacturer controllers which may differentiate a great deal from manufacturer to manufacturer. Add on the casing, profit by manufacturer and then the profit by reseller and we may understand why slc design will remain as the premium SSD for those that can afford it.

Conclusion

Through a very conservative general estimate, the sale of SSDs will go through the roof within the next few months, most resulting from the fury of mlc ssds which will hit the market any time now at a price well below half of the slc ssd. We can now see that, as with auto manufacturers, you will have the option of purchasing the Cadillac or the GM of the SSD. 

As costly as it is, the Memoright 128Gb SSD is now and will remain the ultimate SSD to own, not only because it is the only 128Gb slc design SSD available to the consumer, but because it will outlive and outperform any other consumer SSD on the market. Somehow, Memoright has found a way to push in the capacity that many seek as well as finding the performance that many will pay the premium for.

We are now fully prepared for the onset of the SSD revolution as one can, not only differentiate between a slc and mlc SSD but also, understand the pricing, performance and lifespan considerations of each.

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i7-930@4Ghz/Rampage III Gene w/SATA3/6Gb 1600 TC/MSI N460GTX/Crucial C300 64Gb/OWC Extreme Pro 120Gb/CM Hyper 212/OCZ ModXStream 600w
Acer 1810t-6188/11.6 LED/SU9400@1.4/4Gb/Intel X25m 160Gb/WD Cavier 160Gb/4500mHD/HDMI/BT/Win7Ult64Bt - 7.8 Sec Boot Time 
1. The SSD Review
2. My Computer and Win7 Optimization Guide
3. The Win7 SSD Performance Optimization Guide

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Last edited by Les : 02-25-2008 at 04:10 PM.