Top positive review
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Good but only if you can get it at the right price
on November 6, 2012
First of all, the Samsung 840 uses triple-level cell (TLC) memory vs. the multi-level cell (MLC) memory used for almost all other SSDs. So what is the difference between TLC and MLC? With TLC memory, 3 bits of information (8 possible values) are stored per cell instead of the 2 bits (4 possible values) in MLC memory. This might sound better but it also means that the cells are used more and there is less voltage fault tolerance. In the most simple terms, you can think of cells being "filled" or "emptied" by applying voltage. When 3 bits (8 possible values) of information are stored per cell, the SSD may have to apply voltage to the entire cell multiple times even though just one bit of information is encoded (depending on the bit being changed). The multiple voltage applications to each cell also slows down the write speeds and causes more wear in general.
Performance-wise, the Samsung 840 is actually worse than the previous generation Samsung 830. On the Passmark benchmarks, it is slightly lower than the OCZ Agility 4 which uses asynchronous MLC NAND flash.
Right now, the top three SSDs (best selling and best reviews) on Amazon are the OCZ Vertex 4, Samsung 830, and the Crucial M4. However, all three of these SSDs are cheaper and they perform better than the Samsung 840. Eventually, the TLC NAND should mean that the Samsung 840 will sell for less because less NAND is used. However, it is not worth it right now. To be safe, I would say that you should wait until the drive is at least 10-20% cheaper than the three drives above due to the uncertainty of how it will perform in the long run.
One puzzling thing is that I couldn't find any mean time before failure (MTBF) estimates for the 840, while the MTBF for the 20nm MLC based 840 Pro is 1.5 million hours (lower than the 2 million hours for many 25nm MLC drives). Also, it is interesting that the drives are not 128GB, 256GB, 512GB. There are probably three reasons for this. The TLC NAND manufacturing process has lower yields so they have to "turn off" a portion of the cells to account for this. This is similar to what Nvidia does for their GPUs (16 functional streaming multiprocessors become GTX 580, 14-15 functional streaming multiprocessors become a GTX 570, etc...) Another possibility is that they anticipate that there will be more wear to the TLC NAND so that there is an unused memory bank to replace any dead cells. The third and most likely reason is a combination of the two, i.e. a portion of the turned off cells are nonfunctional due to the new manufacturing process and the remaining portion of the turned off cells are reserved for replacement.
I handle tech purchase decisions for my department and so I bought one Samsung 840 to test for due diligence but won't buy more until they become much cheaper.
As predicted, these drives dropped in price quite a bit. TLC NAND is about 25% cheaper than MLC NAND so that the price of these drives should converge to a ratio of around 75% of the price of a MLC SSD in the long run.