Comment 8 for bug 1708572

When I launched an instance using a volume stored on IP-SAN, 'Disk of instance is too large' exception was raised, and the amount of disk space available was also a negative number. It seems that all allocated IP-SAN volumes used to create instances are taken into account when computing the free space on the target host.

The fix in https://review.openstack.org/524681 does an over-committed check, but I'm not sure it would help if we launch an instance from a Cinder storage backend.