Oversubscribe with live-migration

[Aseel Awwad]

The live migration operation for a VM with a number of VCPUs larger than the free VCPUs on the destination host(for example, a VM with 40 VCPUs to a host with 2 free VCPUs) was done successfully althoug cpu_allocation_ratio is 1. While the creation of 40 VCPU VM fails with Insufficient compute resources: Free vcpu -2.00 VCPU < requested 40 VCPU. It seems that the live migration process doesn't check for the available resources and compare them with the required resources. While in the instance create process, there is a check for that.

[Aseel Awwad (aseelawwad)]

Output of host info after live migration

[melanie witt (melwitt)]

What version of nova are you using? And did you specify a host when you requested the live migration or no? Did you use the 'force' flag?

The compute API reference [1] says:

"Prior to microversion 2.30, specifying a host will bypass validation by the scheduler, which could result in failures to actually migrate the instance to the specified host, or over-subscription of the host. It is recommended to either not specify a host so that the scheduler will pick one, or specify a host with microversion >= 2.30 and without force=True set."

As long as you are not specifying a host and not using the 'force' flag, the request should go through the scheduler validation to prevent oversubscription of the destination host. Please let us know if you are using an older version or nova and what request parameters you used for live migration.

[1] https://developer.openstack.org/api-ref/compute/#live-migrate-server-os-migratelive-action

[huanhongda (hongda)]

I think this is duplicated with these two bugs:
https://bugs.launchpad.net/nova/+bug/1289064
https://bugs.launchpad.net/nova/+bug/1712008