[GTALUG] Compressing an image of a microSD card

[Giles Orr]

I have an Anbernic RG350, a very nice hand-held gaming console:

https://retrogame300.com/products/rg350

It uses a Linux distro that was developed for an older hand-held game
console.  It turns out that the OS and "internal storage" are on an
actual microSD card inside the case: four screws and a gentle tug to
get the card out, and you can image the "main drive."

It's a 16G SD card with two partitions that occupy the whole card.
Between them, they have 1.4G of used space.  I imaged the whole card:

    # time dd if=/dev/sdi bs=4M of=./RG350.SD.2020-05-14.img
conv=fsync status=progress

This produced a roughly 16G image as expected.  Then I compressed it:

    # time xz --threads=5 RG350.SD.2020-05-14.img

This produced a 10G image.

My assumption was that compression would see the empty partition
space, presumably as a bunch of zeroes, and compress the crap out of
it so that the final image would be the same size or smaller than the
1.4G of used space.  I'm aware that the free space may not be zeroed
out ...  It's not encrypted, so that's not the issue.  Is there a sane
way to back this up that would produce a smaller image?  I prefer to
image the whole card ... and I like 'dd' because I'm familiar with it,
although maybe I should move on ...  Is somehow zeroing the empty
space on the card a possibility?  Suggestions welcomed ...

I can live with the 10G backup if I have to, but would prefer a
"better" solution if it's available.

-- 
Giles
https://www.gilesorr.com/
gilesorr at gmail.com