On Sun, 3 Jun 2018 16:29:33 +0000 (UTC), Martin Gregorie
 declaimed the following:


>Some classes of card give poor performance, though that may well be
>related to fragmentation. Don't forget that SD cards were designed for
>large reads and writes (still images, videos...) and not for the type of
>random access needed for good Linux filing system performance, and this
>is probably exaggerated by the RPi's relatively small RAM, which reduces
>its ability to cache data. But, if it doesn't work for you, fair enough.
>
 Classes 2/4/6 are based upon I/O on fragmented file systems as would be
found on still image cameras (small files with random deletions by the
user). Class 10 is based upon streaming a single video file on a freshly
formatted card. Though also take into account that the standard format for
SD cards is FAT/exFAT -- not a journaling system.

 As a result, a Class 10 card could have atrocious behavior when working
with multiple small files. If the file system is read-only, it may not be
that noticeable, but if the file system is undergoing updates (log files,
configuration files [current play list position?]) then a card that
handles, say, 2 allocation units at a time will be doing a lot of
closing/opening/copying/erasing of data from allocation unit to allocation
unit -- whereas a card that can handle around 6 allocation units at once
may not need to do rewrites, but can buffer changes to multiple files for
later flushing.

 I really should have bookmarked the article about this...



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