Comment 26 for bug 1469179

The workarounds don't work so well for pre-existing clusters that have a mix of boot-from-volume and ephemeral (non-volume) instances. This is because currently deployed instances all use the same disk flavors and it's no longer a simple fix to introduce different flavors as nova-scheduler already thinks a bunch of disk space is already used up by pre-existing instances backed by cinder when they aren't, overestimating usage. Perhaps changing the existing flavors to have 0 byte root disks would make that go away, but then any instances that actually do run using local storage will be considered to have 0 byte disks as well, thus underestimating usage.

Disabling DiskFilter is really the only option, but that obviously can also lead to issues in mixed clusters because disk is actually important.