sdcardio: Use CMD25 across multiple writeblocks() calls #5510
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This increases write rates (of gifio from adafruit#5490) from about 2.4fps to over 5fps by making more efficient use of the SD card protocol. Because of details of oofatfs, it usually manages 64 writes in a single CMD25, then two writes in a different area of the SD card (presumably, filesystem metadata). I couldn't find where to increase "64" to a higher number. This may depend on the allocation size of the filesystem. I tried preallocating too, but oddly it significantly lowered the write rate. Any trailing data is committed when the file is close()d, or when the `sync` method of the SDCard object is called.
tannewt
requested changes
Oct 25, 2021
This was referenced Mar 25, 2022
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This increases write rates (of gifio from #5490) from about 2.4fps to over 5fps by making more efficient use of the SD card protocol.
Because of details of oofatfs, it usually manages 64 writes in a single CMD25, then two writes in a different area of the SD card (presumably, filesystem metadata). I couldn't find where to increase "64" to a higher number. 512*64 = 32768 bytes is the cluster size of the filesystem.
I tried preallocating too, but oddly it significantly lowered the write rate.
Any trailing data is committed when the file is close()d, or when the
syncmethod of the SDCard object is called. You can check that this is happening by looking for the same token on the first & last lines of the file.Test code (no gifio required):
code-cmd25.zip
Test board: Kaluga (v1.3), 32GB "class 10" micro SD card with "G.SKILL" branding in a https://www.adafruit.com/product/254
Speed in the test program increased from about 260KiB/s to over 725KiB/s.