when an instance is launched,
nova/compute/manager.py: _run_instance() checks the size of the instance to run against the instance-type to see if it will fit.
In libvirt, what it really does is check the size of the image against the size of the ephemeral disk (local_gb). If local_gb == 0 (m1.tiny), then this check will let *anything* through.
nova/virt/libvirt/connection.py is consistent in its non-checking of any size for m1.tiny, but worse, it does it explicitly based on name == 'm1.tiny'.
Comments like the following in manager.py indicate that 'local_gb' is actually supposed to be the size of the root disk, which seems to make more sense:
# NOTE(jk0): Since libvirt uses local_gb as a secondary drive, we
# need to handle potential situations where local_gb is 0. This is
# the default for m1.tiny.
The result of this is that in libvirt:
* the root disk size is actually determined by the flag 'minimum_root_size' which defaults to 10GB.
* [not verified with test] any image size can be run on an m1.tiny, the image will be neither resized up or down
* [not verified with test] for all other types (where local_gb != 0) the image size is checked against 'local_gb', not against the size
libvirt needs to start using local_gb in an instance_type size to be the root disk, rather than ephemeral0.
This may have been adjusted by "KVM and XEN Disk Management Parity"
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