I understand how to use the backup client itself, what I am looking for is a good suggestion of which directories to include and exclude in a Proxmox Host backup. If I'm already backing up the VMs/CTs, then obviously I won't need to backup the /var/lib/vz/images directory, but what is the optimal set of directories to backup for a PVE server?
Also, I can't tell if backups created with separate .pxar images from the same host count towards the retention policy. That is, if I run a backup to create etc.pxar of the /etc directory and a separate one to create opt.pxar of the /opt directory, does the retention policy treat those both as two backups towards the retention policy, or one of each based on the name? The GUI makes it seem like they both count together, but it seems like a better approach would be to lump them under the host, but then group them by the pxar name used in the backup.
I just wanted to follow-up on my second question above. If you use the --backup-id <string> parameter in the client backup, then PBS sees it as as different backup set named for this parameter. By default it uses the hostname without this parameter. Using that parameter allows a host to generate different backup sets with the retention policy applied to each separately.
yes, your interpretation is correct - each snapshot (combination of type, id and time) counts, the contents are irrelevant for that purpose. note that you can store multiple pxar archives and blobs in a single snapshot, so it's possible to backup '/' as root.pxar, '/etc' as etc.pxar and '/home' as home.pxar all in one snapshot for example.
in general /etc including /etc/pve is probably the most relevant part of a PVE host to save (beside guests, obviously ). you might also want to take care of templates, snippets, etc. from your directory storages depending on your usage. /var might also contain interesting things (like logs, but also the actual sqlite DB backing /etc/pve), as well as the home dirs (/root and /home/...).
since PVE uses Debian under the hood and allows a lot of flexibility/customization, it's a bit hard to give advice fitting all systems out there when in doubt I'd rather backup more (and exclude specific cache/tmp dirs that are completely uninteresting) than less.