So, I started looking inside my snazzy EX2 ultra I got to replace my damaged Gen2 (cats on shelves are adorable, until they knock expensive disks over!), just to see what additional kernel modules it has baked into its cramfs blob. Turns out, it supports btrfs. (shock!)
Why is that exciting? Well, btrfs supports seamless in-place filesystem compression with a variety of compression algorithms, one of which is zstd. In prior tests of this FS, I determined that it gets about double the compression efficacy of NTFS file compression, and does it more sanely too. (No scattered file extents all over the darn place.)
Since the EX2 and the Gen2 single bay units both use the same exact kernel, it should be possible to re-bake the crafms container to add the missing module, and poke the init script at the same time to make it load.
More fun experiments to try tomorrow!
Looking at the hd_info.xml file in /usr/local/config, it has an entry for filesystem type. I think I will try re-baking the cramfs container as outlined above, and then testing if I can get the main volume to use btrfs on the newly revived gen2. Since I have adapted it to use flash memory, it’s capacity is going to be very small (only 1TB), but turning compression on should do wonders.