design - What gives some flash memory a higher speed rating than other flash memory?

What gives some SD/MMC cards a much higher speed class rating than other SD/MMC cards?

Why are some solid-state "disks" (SSDs) much faster than other SSDs?

I'm hoping that I can take at least some of ideas for storing data faster, and apply those ideas to a data logger design I'm working on that stores (often bursty) data to a few flash chips.

(Ideas that involve re-designing flash memory cells and fabricating new flash chips to work faster are interesting, but are not as useful to me since, alas, I don't own a fab).

Answer

In addition to the fact that some flash devices are capable of writing more bits in parallel, another factor affecting speed is the way in which garbage-collection is performed. One of the biggest sources of slowdown on flash drives stems from the fact that most flash devices do not allow 512-byte pages to be erased and rewritten; instead, they require that erase operations operate on much larger areas (e.g. 32KB or more). If a device is asked to rewrite block 23, it will find an empty page, write "I am block 23" along with the new data, then find the old block 23 and mark it as invalid. If the number of empty pages gets too low, the device will check whether there's any erasable block which don't hold any valid pages. If not, it will find one which has very few valid pages, and move each page to a blank page in some other block (invalidating the old ones as it goes along). Once a block has been found which doesn't have any valid pages, that block can be erased, and all its pages added back to the pool of blank ones.

Many schemes can be used to keep track of how pages are mapped and determine which blocks should be recycled when. It's possible to design fairly simple schemes that can be implemented on a small micro with limited RAM, but performance may not be great (e.g. it may have to repeatedly read through the flash to identify blocks for garbage collection, and may place data blocks without regard for whether they're likely to become "obsolete" soon). Conversely, if the controller has a generous amount of RAM available, it may be able to do a better job of identifying which blocks should be garbage-collected when, and may also be able store blocks of data with other blocks that will have similar useful lifetimes.

Incidentally, I consider it unfortunate that solid state drives have not standardized on some sort of file system at the controller level (meaning that rather than asking for block #1951331825, software would ask for blocks 4-8 of file #1934129). A flash drive which knew how information was stored in files could make much better decisions about which data should be placed together than one which simply has to deal with seemingly-independent writes to various sectors, and could also do a more effective job of ensuring data integrity under adverse conditions.