Cool => I'll then probably reformat on my root server the partition hosting the images of my VMs from EXT4 to XFS => then to make 100% backups from time to time I would just have to shut them down for a minute, do a "cp --reflink" of their image-file on the host/dom0, then I could start them up again and do anytime later the slow download of their "copied" file.

Any recommendations about a utility (working on a layer higher than the filesystem) that can synchronize only the chunks of a specific file that has changed since the last sync and which generates as final result a "normal" file (no behind-the-curtains database - just copy the identical data from the local/old file and the changes/new data from the remote location to get a normal up-to-date local file)?

I believe rsync is the utility you are looking for.

Damn, now I'm ashamed of myself - it's since forever that I'm using rsync... :)

I think that when I tested this with rsync a long time back I did a similar mistake like this one https://stackoverflow.com/questions/28819379/block-level-cop... (doing a sync not over a network) and/or this one https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_l... (doing a sync by adding/removing some bytes instead of just changing their values, as it would happen in the preallocated img-file of the VMs).

I'll test it again - thanks a lot for the info!!

Yea my memory was fuzzy and I knew it was planned and being worked on but couldn't remember if it had happened yet. I think F2FS and bcachefs are also supposed to support it but I'm not sure about their level of support either.