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.


--
 Wulfraed                 Dennis Lee Bieber         AF6VN
 wlfraed@ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/

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