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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.
> 

You're right. A filesystem is indeed created on the backing file. Sorry :)


> 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