I have love/hate relationship with Proxmox Backup Server (PBS). It's a groundbreaking product, cutting edge technology, entirely free, and a complete pain in the ass. Because of all of the above, I've spent a good deal of time haunting the user forum, irritating the staff, and spreading unpleasant truths about PBS.

So, what's amazing about Proxmox Backup Server? It's the realized dream of complete dedupe. There are a couple similar products out there, but largely whatever you think you know about backups probably only barely applies to PBS.

PBS should be deployed on SSD, because its really a huge database of file chunks. (Literally, they are called .chunks.) PBS scans its .chunks to see if it already has a .chunk, and if it does, it is not copied. Absolute dedupe, every time. A backup job consists of a whole lot of reading and calculating, but as little writing as possible. The actual backup itself, such as it exists, is a set of metadata that says which .chunks will be ne