Comment 5 for bug 1322179

Actually there already is a filter in Nova scheduler that does almost exactly that (except those extra 2 MiB) - ExactDiskFilter [1].

Moreover, it is included in default set of filters for IronicHostManager [2], but devstack (and probably others) do not use these default filters by default (sic) [3]

So probably the solution would be to update the ExactDiskFilter to account for extra 2MB (or create a modified variant of it) and start actually using it in scheduler filters.

[1] https://github.com/openstack/nova/blob/master/nova/scheduler/filters/exact_disk_filter.py#L23
[2] https://github.com/openstack/nova/blob/master/nova/scheduler/ironic_host_manager.py#L39
[3] https://github.com/openstack/nova/blob/master/nova/scheduler/ironic_host_manager.py#L45