KVM fixed memory - ballooning?

[hansm]

In PVE 4.4 when creating a new VM, at the tab Memory there's a new checkbox "Ballooning" when "Use fixed size memory" is selected. I thought ballooning is only used with dynamic memory assignment (https://pve.proxmox.com/wiki/Dynamic_Memory_Management#Ballooning). Also the "Help" button doesn't explain the option for fixed memory.

Why do we have this new checkbox? Maybe an update of help and wiki pages is needed?

 

[hansm]

Update: OS selected: Linux 4.X/3.X/2.6 Kernel

 

[dcsapak]

the ballooning checkbox indicates if the vm gets the virtual balloon device.
in fixed memory mode, qemu sets no target size of the memory, so it handles it automatically.

if you choose automatically allocate memory in range
we set the target memory value and qemu behaves like the description in the documentation

but yes, thanks this is missing in the docs

 

[Mecanik]

in fixed memory mode, qemu sets no target size of the memory, so it handles it automatically.

Can you elaborate that ? Because it really doesn't make any sense, "fixed mode" but it's still dynamic...

 

[dietmar]

Because it really doesn't make any sense, "fixed mode" but it's still dynamic...

The driver is able to report memory usage, so it makes sense to load it, even if it does not handle memory dynamically.

 

[mjw]

So fixed memory size means the memory is never available for the host or other VMs, even if ballooning is on and the VM doesn't use the memory itself? What are the reasons for using fixed size memory (as the default)?