Originally Posted by wdgann
Here is a test from newegg:
it shows the ssd cache can really improve program speed after its analysis you use it often and push in ssd cache. Well 8GB is ...kind of not much. I saw somewhere someone said this 8GB is "higher quality" kind of nand, let's see if anyone knows this and explain more.
I'm not sure what they mean, but I can guess:
There are three main types of NAND flash. The first is SLC, the second MLC, and the third TLC. SLC stands for single-level cell. Each cell can trap a single electron, and that determines whether a bit is a 1 or a 0. MLC was coined when consumer SSDs became cheaper, and stands for multi-level cell. Each cell holds two electrons, which determines two bits. TLC is the newest and is only found in the Samsung 840 and 840 EVO as far as I can tell. It stands for triple-level cell, and, you guessed it, can hold up to three electrons determining three bits.
TLC is cheaper than MLC which is cheaper than SLC, but SLC lasts longer than MLC which lasts longer than TLC. When a write is made, an electron is either trapped or dumped. Sometimes, they get stuck. When that happens, the cell can be read, but not written to. More electrons per cell means more chances for them to get stuck. They just wear out over time, while ReRAM (resistive random access memory) can both retain information and be written to I think theoretically an infinite number of times.
Finally, the write speeds for all three types are different. The Samsung 840 and 840 Pro use the same controller, and the read speeds are about the same. The 840 is TLC, and the 840 Pro is MLC. The write speeds of the 840 are about half that of the 840 Pro for the smaller capacity drives and about three-fourths for the larger drives. The 840 EVO, despite being TLC, has a cache that emulates SLC. 9GB of NAND are taken out of use for the 120GB and 250GB models and are used as a 3GB pseudo-SLC buffer. The write speeds when using the buffer are on par with the 840 Pro, and both approach the SATA III speed limit.
So I assume they mean an SLC cache is used for the hybrid drive.