On Sat, 22 Feb 2020 19:03:55 +0100, "Wendelin Uez" 
declaimed the following:

>Can one detect such an optimized class 10 card?
>
 Unless one can obtain detailed specifications for the chip(s) inside
the card, I suspect the only means is by benchmarking. Note that many of
the lower cost cards are distributed by companies that don't produce chips
-- they contract out with the few chip foundries for a lowest-bid batches,
and when they run out of a batch, the next batch of "the same" may come
from a different foundry with different behavior.

 Unfortunately, the most detailed information source became a dead link
a year or so back -- it had a table showing how many allocation units could
be held open at once, along with performance on read/write of streams and
small random files.

 The second source is still alive: https://lwn.net/Articles/428584/
"""
Restrictions on open segments

One major difference between the various manufacturers is how many segments
they can write to at any given time. Starting to write a segment requires
another physical segment, or two in case of a data logging algorithm, to be
reserved, and requires some RAM on the embedded microcontroller to maintain
the segment. Writing [SSD thrashing] to a new segment will cause garbage
collection on a previously open segment. That can lead to thrashing as the
drive must repeatedly switch open segments; see the animation behind the
diagram to the right for a visualization of how that works.

On many of the better drives, five or more segments can be open
simultaneously, which is good enough for most use cases, but some brands
can only have one or two segments open at a time, which causes them to
constantly go through garbage collection when used with most of the common
filesystems other than FAT32.

When a drive reserves the segments specifically to hold the FAT, these will
always be open to allow updating it while writing streaming data to other
segments.
"""
{Note the emphasis on FAT file system}

 A more recent entry:
https://www.cnx-software.com/2019/08/16/wear-estimation-emmc-flash-memory/
(since wear is caused by erase cycles, and cards with a low open allocation
count could trigger more erase cycles, such cards will wear faster, and
likely be slower in operation). There tool is currently in "public Alpha
release" https://labs.toradex.com/projects/flash-analytics-tool BUT... only
appears to work with some of their products (Apalis and Colibri boards) --
pity, as the "block level erase count" monitoring might have shown
significant differences between the cards if comparing start and end counts
during the image flashing.


 An R-Pi specific page -- which includes instructions for duplicating
the benchmarks: https://www.pidramble.com/wiki/benchmarks/microsd-cards
"""
Most cheap microSD cards, even if rated as being 100MB/sec+ class 10 cards,
can’t sustain anywhere near that rate when writing random data—especially
on the Raspberry Pi’s measly data bus. (Note that most of the above
benchmarks, when run on a USB 3.0 card reader on my MacBook Air, show 5,
10, or 15 times greater performance in that environment).
"""

{Hmmm, wonder if that benchmark will also run on the BeagleBones}




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