On Sun, 22 Jul 2018 18:57:30 GMT, Jan Panteltje 
declaimed the following:


>Do not have a free card ATM, but here the read timings for a 8 GB raspi card
on my Core5 laptop, card in /dev/sdc
>

 I wouldn't consider read timings to be of significance. As has been
mentioned, SD cards -- to write to a block -- have to have an erased (all
1s) block available. However, erasure takes place in allocation units which
are larger than file-system blocks. So before a "random" write can occur,
the card has to erase a complete allocation unit, copy over data "before"
the block to be written, write the new block, and if the I/O now shifts to
another random block, copy the rest of the old allocation unit into the new
one -- then put the old allocation unit into the free list.

 Class-10 cards are rated for streaming a single (video) file to a
freshly formatted card, and may be totally trashed by small random (photo
creation/deletion) files. Oh, and that is FAT file system -- just the open
file and the open FAT area, for two allocation units at a time.

 The better cards (and most Class-2/4/6) are capable of maintaining
(buffering) 4-8 open allocation units, which much better supports random
I/O and usage of non-FAT (eg; journal ling) file systems. Things like
alternating writes to two files -- both files can be in separate open
allocation units without triggering unit closure, erasure, rewrite...



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