The advice in the manual for the balloon setting to prefer virtio-mem over ballooning is hypervisor-dependent and not quite right in general; or at least, it is confusing the way it is stated. For example, QEMU uses the balloon device to implement free page reporting. Free page reporting is useful even if the balloon never inflates. Even if using virtio-mem, which itself can have its pages freed, one still needs the ballon to create the communication channel for that, AFAICT.
It's true that virtio-mem is a better method for hot-plugging memory than ballooning, but that's not its only job.
The advice in the manual for the
balloonsetting to prefer virtio-mem over ballooning is hypervisor-dependent and not quite right in general; or at least, it is confusing the way it is stated. For example, QEMU uses the balloon device to implement free page reporting. Free page reporting is useful even if the balloon never inflates. Even if using virtio-mem, which itself can have its pages freed, one still needs the ballon to create the communication channel for that, AFAICT.It's true that virtio-mem is a better method for hot-plugging memory than ballooning, but that's not its only job.