Unable to Start Instance With Error "Cannot allocate memory"

Problem

Starting an instance from a SHUTDOWN state results in the following error being observed in the pf9-ostackhost log located under /var/log/pf9 directory.

2021-10-11 01:59:23.636 ERROR nova.virt.libvirt.driver [...] [instance: 7e008ee7-3dc1-4a02-b9a4-587d2f9ea591] Failed to start libvirt guest: libvirt.libvirtError:internal error: qemu unexpectedly closed the monitor: 2021-10-11T05:59:23.408060Z qemu-kvm: cannot set up guest memory 'pc.ram': Cannot allocate memory

Environment

• Platform9 Managed OpenStack - All versions
• Nova
• Libvirt

Cause

The instance fails to start because the libvirt process is not able to allocate the required memory being requested by the instance.

Resolution

1. Try migrating a few running instances to a different host.
2. If there is no hosts available to accommodate the running VMs, provision additional physical memory on the host.