The Arch installation tutorial I followed originally advised using LVM to have separate root and user logical volumes. However, after some time my root volume started getting full, so I figured I would take 10GB off of my home volume and add it to the root one. Simple, right?
It turns out that lvreduce --size 10G volgroup0/lv_home doesn’t reduce the size by 10GB, it sets the absolute size to 10GB, and since I had way more than 10GB in that volume, it corrupted my entire system.
There was a warning message, but it seems my past years of Windows use still have me trained to reflexively ignore dire warnings, and so I did it anyway.
Since then I have learned enough to know that I really don’t do anything with LVM, nor do I see much benefit to separate root/home partitions for desktop Linux use, so I reinstalled my system without LVM the next time around. This is, to date, the first and only time I have irreparably broken my Linux install.