One instance where rsync's delta merge comes up for me: For all of my virtual machines, I build a bootable live ISO so that way I can cleanly swap out the full ISO with a new one when there are software updates. This means that every time I want to change a config file inside the virtual machine, I end up building a new ISO and rsyncing it to my server. rsync speeds this up significantly since most (but not all) of the ISO is unchanged.

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greaber 2 hours ago | parent | next [-]

Nice example. Rsync might be better here. Although syq will still avoid copying most of the image if changing the config file just modifies a small number of locations and doesn't globally move data around (like prepending a single byte to the ISO)

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dwedge 3 hours ago | parent | prev [-]

How do you handle updates this way? Do you mount the iso, update and keep it as an iso? Are all of the virtual machines mostly read only?

	▲	craftkiller an hour ago \| parent [-]
		The iso itself is read-only. When I need to update software I build a whole new iso and replace the existing one. For any folders/files that need persistence across reboots, I store that data separately from the iso, either in a virtual nvme drive or a mounted 9pfs folder from the host. The iso mounts the persistence drive/9pfs via /etc/fstab. So in the end, the software and config files are read-only (until I replace them with a new iso) but my databases and data files are read-write. Getting into the unnecessary details: in classic live ISO fashion, the root (/) filesystem is tmpfs. Then I have at most 1 persistence drive and/or at most 1 9pfs mount. From there, I use bind mounts to only persist the few folders/files that I want preserved. For example, I'll have my persistence drive mounted at /persist and I'll bind mount "/var/lib/etcd" to "/persist/var/lib/etcd". Then when I reboot, everything outside of "/persist" is wiped out and I'm back to the initial iso + anything in /persist. So it is very similar to running a docker image with a volume mount, or running tails with persistence. I build it all via NixOS and I use Impermanence to manage the bind mounts to the persist drive: https://wiki.nixos.org/wiki/Impermanence