On Fri, 21 Feb 2020 13:11:48 +0100, "Wendelin Uez" 
declaimed the following:

>> Any ideas about problems with certain sd cards?
>
>The difference may be caused by different software system or by different
>physical card I/O properties (even if the cards claim to have same speed
>class)
>

 Especially if one is comparing Class-10 cards for fragmented file
systems.

 Classes 2/4/6 were rated for small file creation/deletion, and pretty
much random access on fragmented media (still photo cameras, where the user
may shoot a batch, delete a few, shoot more...).

 Class 10 cards were rated for streaming a large single file onto
freshly formatted media (video).

 As a result, some Class 10 cards are optimized (cheapened) to only
handle the drive allocation bitmap and one open file (FAT file system)
while Class 2/4/6 cards could manage multiple open files at the same time.
The former cards only buffered two "allocation units" (the chunks that the
card maintains, and which must be erased before writing -- bigger than file
system clusters/sectors) -- moving from one (output) file to another closes
the open allocation unit, finds a free unit, erases it to all 1s, copies
the non-affected part of the file system data to it and then writes new
data. Multiple unit cards can track the bitmap and more than one active
allocation unit without triggering an allocate/erase/copy cycle.

 Journaling file systems are deadly on the "optimized" cards -- since
one has the bitmap, journal file, data blocks for file, and then the
purging of the journal when the OS confirms the file system is up-to-date.


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

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