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Closer Look at Samsung 840 250 GB

As manufacturing technologies get better, synchronous MLC NAND flash memory has made inroads in relatively inexpensive SSDs. We mean popular flash memory types whereas the Samsung 840 Pro uses rather rare 21nm MLC NAND flash with Toggle Mode 2.0 interface, which is not inexpensive at all. The Pro version of Samsung's new SSD may only be marketed in the same price category with flagship SSDs from other brands. It is not a mass-market product. That's why Samsung has also prepared an affordable modification as well. The basic Samsung 840 features the same hardware platform but MLC NAND is replaced in it with cheaper TLC NAND (Triple Level Cell) flash which stores not two but three bits of data in each memory cell.

TLC NAND is not an innovation. It is used widely in USB flash drives. However, it is the first time we see it in an SSD, so the Samsung 840 is kind of experimental. TLC NAND is cheaper than the popular MLC NAND and also inferior to it across many parameters. Particularly, it has lower access speed and, which is even worse, shorter service life. The following table illustrates this fact:

TLC NAND flash can store not two but three bits of data in each memory cell by increasing the number of electrical charge levels on the transistor’s floating gate from four to eight. However, it becomes harder to recognize the signal and programming such memory cells requires higher voltage which leads to faster deterioration in the semiconductor design. The higher bit density results in smaller physical size. MLC NAND needs 1.5 times more transistors to store the same amount of data as TLC NAND flash. And since the transistors are actually the same, TLC NAND flash is supposed to be 1.5 times cheaper than MLC NAND flash.

Using TLC memory in inexpensive SSDs is an attractive opportunity but no one has ever tried to do that as yet. The manufacturers and users have apprehensions about its low service life which is declared to be 1000 program/erase cycles. However, a simple computation can show that, assuming a realistic write amplification coefficient of 3x, saving 20 GB of data to a TLC-based 256GB SSD daily will only make it fail in 11 years. A 128GB SSD will last only half that time, but it is still more than enough for a consumer-class SSD.

So, the Samsung 840 is a compromise. It is theoretically not as long-lasting as SSDs with MLC NAND flash but, on the other hand, it represents an inexpensive version of one of the fastest SSD platforms available. Samsung is quite confident about its reliability, shipping this inexpensive SSD with a standard 3-year warranty.

The Samsung 840 doesn’t differ much from its Pro cousin in accessories or appearance. The manufacturer obviously doesn’t want to emphasize its entry-level positioning or inferiority, so it looks as noble as the Samsung 840 Pro. Its box is just slightly different in design, using gray as the predominant color. The picture on the front of the packaging shows the SSD en face rather than in an isometric view.

 

Like the Pro version, the basic 840 comes with some documentation and a CD with electronic manual and software. There is no adapter to mount it into 3.5-inch system case bays. There are also no frames or something to increase its thickness from 7 to 9.5 mm.

The SSD looks identical to its senior cousin, so you can only differentiate the 840 Pro and the basic 840 by the info sticker on the bottom surface of the unified case.

 

Considering that the Samsung 840 uses the same hardware platform as the flagship model, there's a familiar PCB inside but some of the chips populating it are different.

 

The memory chips are labeled K9CFGY8U5A-CCK0 rather than K9HFGY8U5A-CCK0, so it is TLC NAND, even though the chip design is the same. Each chip contains four 21nm semiconductor dies with Toggle Mode 2.0 interface. There are eight chips in total, each with a capacity of 32 GB and the Samsung 840 uses 4-way interleaving on each controller channel.

Otherwise, the Samsung 840 has the same components as its senior cousin including a 512MB LPDDR2-1066 SDRAM chip for cache (it is labeled K4P4G324EB-FGC2). The controller is the same, too. Thus, the Samsung 840 retains the key advantage of the Pro version, its high-performance triple-core processor. The lower specified read and write speeds are only due to the higher-latency flash memory.

Here is a summary of the Samsung 840 250GB specifications:

  • Controller: Samsung MDX;
  • Interface: SATA 6 Gbps;
  • Flash-memory: synchronous 21 nm Toggle Mode 2.0 TLC NAND;
  • Size: 250 GB / 233 GiB;
  • Cache-memory: 530 MB LPDDR2-1066 SDRAM;
  • Sequential read speed: up to 240 MB/s;
  • Sequential write speed: up to 520 MB/s;
  • Random read speed (4 KB blocks): 95,000 IOPS
  • Random write speed (4 KB blocks): 44,000 IOPS.

Judging by the official specs, the slower TLC memory affects write speed in the first place, the Samsung 840 being only half as fast as the Samsung 840 Pro at writing. The read speeds don’t differ that much. One more thing to be noted is that the Samsung 840’s storage capacity is 6 GB smaller because the reserve pool is larger at 9% of the total capacity. This may help to make up for the shorter service life of TLC memory.

Although the Samsung 840 is an inexpensive product, its manufacturer didn’t deliberately strip it of extra features. Like the Pro model, it supports AES encryption with a 256-bit key and is compatible with the exclusive Magician utility.

In other words, the Samsung 840 offers the same functionality as the Pro model. The difference is in performance and, theoretically, in service life, which is reflected in the shorter warranty period.

 
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