In addition to snapshots of the filesystem, I like to keep a copy of the packages I installed. Fortunately, the package managers of Debian and Archlinux do that by default. After some time, however, this cache occupies a lot of hard disk space. I've thus moved the cache to an external drive for my Mini, but I also don't want to waste precious SSD space on my Desktop just for static packages kept for being on the safe side. How to do that on Arch?

Very simple:

CacheDir = /your/favorite/path