On Fri, 04 Dec 2020 13:01:36 -0500, Dennis Lee Bieber wrote:

> On Thu, 03 Dec 2020 21:59:13 GMT, alister 
> declaimed the following:
>
>>not only are they not an exact measure some manufactures either do not
>>know what they mean or simply lie (I have seed cards claiming class 10
>>but then in the specifications quoting transfer rates that were barely
>>class 2!)
>>
>>
>>
>  One: all the SD card ratings assume one of the FAT file systems
is in
> use. That means no journaling of file changes to the media.
>
>  Two: Class-10 rating is based on streaming a SINGLE file (video)
on
> freshly formatted card -- absolutely no file fragmentation. Class-2/4/6
> is based upon multiple small files (still image photos) on a possibly
> fragmented (if one has deleted some photos but not all) card.
>
>  Three: Card makers support differing numbers of "open allocation
> units". The cheaper Class-10 cards may only have two open AUs -- one
> holds the FAT and the other buffers the single file data. Better cards
> may support up to 6 open AUs. Since flash memory requires a full erase
> (to all 1 bits) and then can write 0-bits to the block, but can not
> convert a 0-bit back to a 1-bit without doing the entire block, every
> time the card has to jump to a different block it has to perform an
> erase and merge of "old" data from a different block, before writing new
> data into the block. Having 6 AUs allows the card to keep some blocks
> open for random I/O access without committing them to the flash memory
> and running an erase cycle. This really helps for journaling file
> systems, since any write ends up with changes to at least three blocks
> -- write data , write meta-information to journal, sometime
> later commit journal to actual file system meta-data. On a 2 AU card,
> every toggle from write data (presume, say, a log file that gets a new
> line every so often) to update journal would trigger an erase operation
> on the card.

yes but the card I refereed to not only tested as slow transfer rates the
makers small print also quoted the same (class2) data rates.
the class 10 branding on the card was blatantly false.

The obscure/cheap brands are simply not to be trusted


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