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InfluxDB
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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.