Does process substitution actually wrote the content to disk first or not? The information on internet I found seem to be conflicting on this. If it's actually writing the data into disk first, then it's probably won't solve my problem (limited disk i/o). Afaik writing to pipe won't result in saving the data to disk temporarily. I guess the only way to know is to try it out on my system and see how it performs.
If process substitution doesn't work, shouldn't /dev/stdin work? I haven't tried it, but as long as b2 doesn't try to check the file size before uploading I don't see why it wouldn't work: b2 upload_file bucket /dev/stdin < file
It definitely doesn't write it to disk first. It's basically a pipe() under the hood, but exposed as a file descriptor. Downside is that seeking doesn't work, but that shouldn't affect your case.
