Previously, a combination of circumstances could result in a failed live migration. This could arise when using block migration, with disk overcommit enabled, and using a client sending microversion < 2.25 (this version scope included the openstack client, but not the nova client). As a result of these circumstances, the live migration call could fail with the following error:
Migration pre-check error: Unable to migrate [instance uuid]: Disk of instance is too large(available on destination host:[some negative number] < need:[disk space required])
With this update, the check has been updated to correctly consider overcommittal of existing instances. As a result, live migration succeeds as expected.