[tor-relays] PSA: exclude /var/lib/tor/diff-cache/ from your backups

Hello,

Turns out that dir is highly variable, and judging from the name, also disposable.

In my case it was responsible for about 20 GB of churn over a month, i.e. it took 25 GB to keep incremental backups of two Tor nodes with only 2 GB each in root FS (and I was wondering what's going on with my backup location's free space...)

Dear Tor project: this is very bad style to keep that in /var/lib, content with properties like this must go into /var/cache/ [1], where it would be automatically excluded from backups by default policies in most backup systems.

[1] http://www.pathname.com/fhs/pub/fhs-2.3.html#VARCACHEAPPLICATIONCACHEDATA