The default size limit for a snapshot for a VM is set to be one terabyte in Openstack.
An Instance disk size exceeded one terabyte size limit.
If VM root disk is image add image_size_cap parameter using 'viocli update glance' command.
The size value is specified in bytes.
Example for increasing the size limit to three terabytes shown below
viocli update glance
conf:
glance:
DEFAULT:
image_size_cap: "3298534883328"