Comment 2 for bug 1546224

This can only be caused by running Galera in multiwriter mode (probably unintentionally, due to https://bugs.launchpad.net/fuel/+bug/1490595).

Anyway, this should be a really rare case, as usually instance_info_cache_update() is called only from 1 nova-compute at a time, except for the case when it can be updated from nova-api on association / disassociation of a floating IP.