The network block device (NBD) protocol was first introduced 23 years ago (Linux 2.1.55; April 1997), as a way to access block devices over a network.

In ~2002 I scraped together enough money (in college at the time) to replace my desktop. I bought a case/motherboard/cpu, but didn't have enough to buy lots of memory for the new machine (which didn't use the same memory as my old one), so I had 128MB on the new one and ~800MB cobbled together from various dimms on the old one.

Then I read about NBD. Half an hour later I had made a 640MB ramdisk on the old machine and mounted it with NBD on the new machine, and used it as a swap disk. I couldn't believe how well it worked. Fun times.

Great story.

After you pointed out the NBD protocol I went and looked it up. Wish I'd learned about it a couple years ago!

Found this gem on the libguestfs readme:

Well-documented, simple plugin API with a stable ABI guarantee. Lets you export “unconventional” block devices easily.

I really want to know what they think "unconventional" block devices are. Could lead to some interesting projects!

If you're interested in this kind of thing, 9p is also used for exporting storage over the network. https://en.m.wikipedia.org/wiki/9P_(protocol)