I submitted a bug for this question: https://bugs.launchpad.net/nova/+bug/1414947
This bug's status is wishlist, But I have a different opinion. I have a use case: Users may create some virtual machines for the processing of their business, these virtual machines use customized images which can meet their requirement. When users create virtual machines using these customized images in batches, they don't need to know the size of instance's root_gb, so they can use flavor with its disk size as 0. In this case, the value of root_gb's filed in db table of instances is 0 for every virtual machine, but it's real value is not 0. This will lead to live migrate failure since live-migration's destination compute node's disk_available_least is always less than 0. We often enable disk_filter to control compute nodes's disk space, but when the value of root_gb's filed in db table of instances is 0, this filter doesn't work. So, I think the value of root_gb's filed in db table of instances should be equal to image's virtual size in this use case.
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