ramroot VS zram-init

Now I can use ramroot and run the whole OS on RAM.

Anyway, I wrote and maintain a guide to make this setup: https://mags.zone/help/arch-usb.html. I also made a package to optionally load everything to RAM during boot: https://github.com/arcmags/ramroot.

I wrote a tool to load root entirely into RAM on Arch. It checks available RAM and prompts on boot.

You could also use the Arch Linux guide above and install Arch Linux on the USB drive, making sure the boot and root partition sizes are less than the amount of ram in the machine you are booting on. Then install ramroot which will load up your system into ram on boot. This will mean the system runs off ram and not USB. If you need to make changes, install applications or update, you could boot it normally so it runs off USB as any changes won't be saved if it is booted into ram. If your home folder is on another partition on the USB, I don't think that is loaded into ram on boot so you can still save files to it and any personalisations should also be kept between boots.

I found a github issue here that seemed to explain things.

Also, sys-block/zram-init::gentoo Scripts to support compressed swap devices or ramdisks with zRAM (https://github.com/vaeth/zram-init/)

IIRC zswap is zram used for swap, so as long as your kernel has zram support, you can use zswap. You can use zram with zramctl from util-linux. However, there is also the sys-block/zram-init package: it's a wrapper around zramctl which offers an openrc script to automatically create zram devices at boot. It also provides configuration for zswap. Configuration is in /etc/conf.d/zram-init.