Comment 6 for bug 1837256

I suspect that the "Block Migration" box was not checked. When using Horizon, if the instance is boot from image (as in this case), then "Block Migration" must be selected on the "Live Migrate" panel. If boot from volume, then "Block Migration" must not be selected.

If live migration is run from cli, e.g. "nova live-migration <instance-uuid>" then no option is required. This is because more recent nova api microversions (>= 2.25) support the "auto" option which will determine the storage backing and do the appropriate steps. However, as it seems Horizon is using an old api microversion, "Block Migration" must be specified explicitly.

Suggest a re-test.