#27. "RE: SD card usage" In response to In response to 26 Fri 06-Sep-13 10:39 PM by hyphotographer
If you search for an article about FLASH memory from The IET (Institution of Engineering and Technology), you will get really useful information about your SD cards. I read it on a journal published by them a long time ago.
I found this on their website, which requires registration (free) and it mentioned the case of formatting your SD card ...
"It has a lot to do with how efficient the wear-levelling is and the write-amplification, which is how much NAND is actually written to." Write amplification occurs because Flash can only be erased in pages, so if you need to update a file, you may first have to move current data to a new page, erase the old page and then write the updated file all over again.
Wear-levelling is the process of evenly distributing usage across the chip, and it is needed because the way Flash works means that each cell can only stand a certain number of erase cycles before it begins to become unreliable. So the controller must not only re-map bad blocks, it must keep track of empty pages, pre-emptively erase stale pages for re-use once it starts to run out of new ones, and avoid creating usage hot-spots. Incidentally, when you rewrite a page, time and power are saved by not erasing the original immediately; instead it is marked 'stale', and the data written to a new page. This perforce creates opportunities for data filchers and forensic analysts alike.
All this, and Flash's lack of mechanical latency, will make it seem very odd stuff to anyone or anything that expects spinning disk - including operating systems and applications. For example, says Kaneshiro: "You don't need defragmentation or write optimisation - the drive actually works better fragmented! If you defrag, you are messing with the wear-levelling."