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Hi Scott,
 
> The other thing I don't like about it is that, currently (at least in
> libvirt driver), ephemeral disks are cached.  This is because ephemeral
> disks are attached with a filesystem already created on them. That
> allows the user to immediately use the disk without having to do mkfs,
> and the caching of that image saves the host from having to do it on
> every instance launch.
> 
> If you use:
>  ephemeral_gb = (flavor.root_gb + flavor.ephemeral_gb) -
> instance.root_gb and instance.root_gb all of a sudden varies greatly
> amoung instances, then the host will be creating ephemeral images (and
> mkfs'ing them) possibly for every instance.

It is my understanding that the filesystem is created on the COW layer of the ephemeral disks (disk.local) and not on the backing file itself by cloudinit or similar software after instance startup. I'm not sure if there's any data on the backing file itself.
 
Instance.root_gb can be either equal to flavor.root_gb or image.min_disk. So yes, the number of ephemeral backing disks will be equal to the different min_disk sizes. But do the backing files for disk.local actually have any data or just sparse files?

Cheers,
~parthi