Proxmox – Assign more memory to KVM VM

You could use the virtio balloon device to try and return memory to the system but this only helps if the guest OS is not using all of its memory

KVM does need some memory for itself to run, but I believe a lot of this gets swapped out after a while

Also, if you end up running lots of similar VMs make sure you have linux same page merging enabled. This will allow memory from similar VMs to be shared; takes more CPU usage up, but is worth it with lots of VMs. I think this is on now by default in ubuntu

KVM and qemu based VMs are just processes. A disk is a resource assigned to the process. If you add a disk, but you don't restart the process, it will not be detected. Rebooting a VM does not restart it, instead, an actual reboot is simulated. So when you stop/start the VM, it actually kills the VM process and starts it with the new settings.

Sorry, things are more complex of course, and I'm being oversimplistic, but once you know how things work, I find it answers questions better than a "do A then B and things will click"

PS: the newer versions of qemu-kvm have a hotplug feature for disks and NICs