I’m not the best person to query about backups, but in your situation I would do the following, assuming both server and desktop run on BTRFS:

Have a script on the desktop that starts btrfs-receive and then notifies the server that it should start btrfs-send.

You can also do rsync if BTRFS is not a thing you use, but It would either be expensive storage wise, or you would only ever have 1 backup - latest.

You probably want to go into a bit more detail on exactly what you want to backup. Are you talking the entire system, flat files, databases…?

run your backup process every few minutes, if both machines are online, it sends the latest snapshot, if not then it exits. You could reduce your snapshots to once a day, so it only does the data transfer once per day, and any extra backups just succeed.

The downside of this approach is you will need to setup alerting if a snapshot doesn’t get backed up within a window, rather then alerting on a single backup process failing. (i.e. if the server doesn’t have a successful backup within a week create an alert)

You could also do the same thing with rclone vfs mounts with full caching, so you could write locally even when the network is offline, and when you get online it would transfer (but you need to setup your server side alerting again)

I use syncthing to copy important files between pc, phone and proxmox server. Syncthing can be set up with version control so it keeps old versions of files.

Only the proxmox server is properly backed up though. to a proxmox backup server running in a VM on said proxmox server. the encryptred backup files are copied to backblaze using rclone

Not sure if this is what you are looking for, but it works for me.

TLDR syncthing for copies between local machines, and proxmox backup server and backblaze for proper backups

Since space is a major concern, maybe have a look at borg and possibly something like borgmatic on top for easier configuration. Borg does deduplicated backups, so you could do even hourly ones if you wanted without too much extra space depending on how many you want to keep. You’d need to run a borg server wherever you want to store your backups so it’s not a simple rsync over ssh situation but that’s the price you pay for the extra niceties.

As to how, I’d probably use zfs send | receive, any built-in functionality on a CoW filesystem, rsnapshot, rclone or just syncthing. As to when, I’d probably hack something with systemd triggers (e.g. on network connection, send all remaining incremental snapshots). But this would only be needed in some cases (e.g. not using syncthing ;p)

Synchting is the way… Been using it since ever. Easy to setup, works flawlessly, doesn’t get into the way.

I use it for server, PC, laptop and android devices.

… Then you want to backup (Borg, restic…) your synched files of course …

I see syncthing being recommended, and like, it’s fine.

But keep in mind it’s NOT a backup tool, it’s a syncing tool.

If something happens to the data on your client, for example, it will happily sync and overwrite your Linux box’s copy with junk, or if you delete something, it’ll vanish in both places.

It’s not a replacement for recoverable-in-a-disaster backups, and you should make sure you’ve got a copy somewhere that isn’t subject to the client nuking it if something goes wrong.