A compute node reports WARNING of "Unknown base file" error in nova-compute.log.

Issue

• A following WARNING message was observed on a specific compute node, what does it mean?

2016-12-22 19:11:34.784 10692 WARNING nova.virt.libvirt.imagecache [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Unknown base file: /var/lib/nova/instances/_base/473d2697cdac3f71d9252a3e76a1da59071e6c47

• The following messages were seen before and after the message entry.

2016-12-22 19:11:31.781 10692 INFO nova.compute.resource_tracker [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Auditing locally available compute resources for no
de compute005.local
2016-12-22 19:11:32.035 10692 INFO nova.compute.resource_tracker [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Total usable vcpus: 48, total allocated vcpus: 0
2016-12-22 19:11:32.036 10692 INFO nova.compute.resource_tracker [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Final resource view: name=compute005.local
 phys_ram=193153MB used_ram=2048MB phys_disk=835GB used_disk=0GB total_vcpus=48 used_vcpus=0 pci_stats=None
2016-12-22 19:11:32.058 10692 INFO nova.compute.resource_tracker [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Compute_service record updated for compute005.local:compute005.local
2016-12-22 19:11:34.784 10692 WARNING nova.virt.libvirt.imagecache [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Unknown base file: /var/lib/nova/instances/_base/473d2697cdac3f71d9252a3e76a1da59071e6c47
2016-12-22 19:11:34.785 10692 INFO nova.virt.libvirt.imagecache [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Removable base files: /var/lib/nova/instances/_base/473d2697cdac3f71d9252a3e76a1da59071e6c47
2016-12-22 19:11:34.785 10692 INFO nova.virt.libvirt.imagecache [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Base or swap file too young to remove: /var/lib/nova/instances/_base/473d2697cdac3f71d9252a3e76a1da59071e6c47
2016-12-22 19:12:32.823 10692 INFO nova.compute.resource_tracker [req-d1d3bc41-dd2b-452c-81a1-2f9942597c5a - - - - -] Auditing locally available compute resources for node compute005.local