Since iPhone 6 and iPhone 6 Plus was released, there are a lot of complaints from the user saying that the devices will reboot suddenly or freeze suddenly. However, fortunately, this does not happen to all the devices released in the market but only to some units out there in the market.
One of the reasons of this is that certain iPhone 6 and iPhone 6 Pluses is using TLC NAND flash whereas the rest is using MLC NAND flash. The difference is that TLC (Triple Level Cell) is generally slower than MLC (Multi Level Cell) but it can store 1.5 times as much data as MLC. This makes TLC cheaper to produce.
However, some guys over at KBench in Korea recently benchmarked 64GB iPhone 6 with TLC and MLC in quest to determine their behavior difference. The first test that they did is Zero Filling. It is basically a way to overwrite all the data on the storage device with null values (0x00). This will permanently remove data stored in the device and it cannot be retrieved. The data amount that is being written is of 3.4GB. It turns out that MLC NAND flash maintained the transfer rate at 75MB/s for most part of the benchmark whereas the TLC has a higher transfer rate at 200MB/s but until around 26MB mark, it started to drop to about 49MB/s when the data amount being transferred reached about 800 MB.
The end result is that MLC is superior to TLC and Apple should have used MLC to product iPhone 6 and iPhone 6 Plus in order to eliminate these inconveniences. However, rumors have it that Apple will discontinue the use of TLC NAND flash and hopefully it will improve the performance of iPhone 6 and iPhone 6 Plus.