Clarifying dedicated and floating memory

[jldevezas]

This has been confusing me for a while now. Let's say we create a VM with:

memory {
  dedicated = 2048
  floating = 512
}

This will translate dedicated to the memory setting and floating to the the balloon setting (also known as minimum memory in the UI).

Am I wrong to assume that the VM will always have 512 MiB of RAM to use and it can go up to 2048 when it needs it? If this is the case, isn't 512 MiB the actual dedicated memory value? And if 512 is the minimum, why is it called floating? Is it not the opposite of floating?

I mean, even ChatGPT is confused about this. Or am I the one hallucinating?

Can someone please explain the logic of the dedicated and floating config in regard to what actually happens in Proxmox?

[bpg]

Related ticket #634

@jldevezas if you managed to figure it out and make a clear picture of all of this, I'd really appreciate a PR with a docs update 🙏🏼

[jldevezas]

@bpg I think I have some docs lying around that I wrote about this. I'm a bit busy ATM, but let me know where you'd like me to drop those docs, and I'll see if I can open the PR and adapt those for this provider.

Also, from what I recall, you might seriously reconsider changing this through deprecation, as it doesn't represent how it actually works and it's actually detrimental to understanding what you are actually setting—if you do a little testing and look at the memory for the VMs, you'll see what I mean. Maybe keep two ways to configure, for backward compatibility, but mark the older version as deprecated. That's what I would do.

Maybe something like this:

memory {
  total = 4096
  balloon = 2048
}

This way it won't collide. Just print warnings for the older configs.

[bpg]

Thanks @jldevezas, if you can open a PR with docs, or just dump them here that would be great!

The memory management was one of the first things the original provider authors implemented a while back, so I’m not surprised it behaves a bit weirdly, the PVE side may have changed since then.

[jldevezas]

@bpg I couldn't find the docs I thought I had on this topic, but check out the memory and balloon settings on the Proxmox docs here: https://pve.proxmox.com/wiki/Qemu/KVM_Virtual_Machines

Quoting for convenience:

balloon: (0 - N)
Amount of target RAM for the VM in MiB. Using zero disables the ballon driver.

memory: [current=]
Memory properties.
current= (16 - N) (default = 512)
Current amount of online RAM for the VM in MiB. This is the maximum available memory when you use the balloon device.

It also links to this, which might be useful: https://rwmj.wordpress.com/2010/07/17/virtio-balloon/

[jldevezas]

Honestly, reading that last link to the blog post confused me even more.

What matters here is that memory sets the maximum amount of RAM. This is set by dedicated on the provider and corresponds to the value that is reported on the Proxmox UI for the VM.

And balloon sets the minimum amount of RAM. This is set by floating on the provider and corresponds to the amount of RAM that can be taken away from the VM and allocated elsewhere (either the host or other guests, but I don't know for sure if guest RAM can grow beyond the maximum, although this probably happens).

So, using floating is perhaps not that inadequate. It's dedicated that becomes a bit confusing, because with ballooning that value is not really dedicated RAM.