Hello guys, I built a Matrix that maybe can help us see the impact of the changes I am proposing. Here (in this system) the formatting is not great, but you can copy and paste it in a notepad, and everything should be fine. With this matrix, you can see the impact of the patch I am proposing.

I do agree that VMs without AZ can float around the cloud. However, the constraint `cross_az_attach` has to be considered when migration/resize happens. This parameter is already considered in other parts of Nova, but not in the migrate/resize workflow.

| OpenStack VM (server) Action | cross_az_attach | Patch applied #864760 |        Result                                    |
| start                        | false           | no                    | Use the host where the VM was previously created |
| start                        | true            | no                    | Use the host where the VM was previously created |
| start                        | false           | yes                   | Use the host where the VM was previously created |
| start                        | true            | yes                   | Use the host where the VM was previously created |


| resize                       | false           | no                    | Resize will trigger a cold migration.            |
|                              |                 |                       | If the setup has mutiple AZs in the same region, |
|                              |                 |                       | and if these AZs are in the same Cell, any host  |
|                              |                 |                       | of any AZ is going to be used. Therefore an error|
|                              |                 |                       | will happen when the system (randomly) selects a |
|                              |                 |                       | host in another AZ.                              |

| resize                       | true            | no                    | Resize will trigger a cold migration.            |
|                              |                 |                       | If the setup has mutiple AZs in the same region, |
|                              |                 |                       | and if these AZs are in the same Cell, any host  |
|                              |                 |                       | of any AZ is going to be used. No aparent error  |
|                              |                 |                       | will happen; however, latency issues might start |
|                              |                 |                       | to happen as soon as the VM is resized, and might|
|                              |                 |                       | be hard for operators to identify the root cause.|

| resize                       | false           | yes                   | Resize will trigger a cold migration. Therefore, |
|                              |                 |                       | if the setup has mutiple AZs in the same region, |
|                              |                 |                       | and if these AZs are in the same Cell, a host    |
|                              |                 |                       | of the AZ where the VM resides is going to be    |
|                              |                 |                       | used. No error happens as the target host is in  |
|                              |                 |                       | the same AZ as the source host; thus honouring   |
|                              |                 |                       | the configuration `cross_az_attach=false`.       |


| resize                       | true            | yes                   | Resize will trigger a cold migration.            |
|                              |                 |                       | If the setup has mutiple AZs in the same region, |
|                              |                 |                       | and if these AZs are in the same Cell, any host  |
|                              |                 |                       | of any AZ is going to be used. No aparent error  |
|                              |                 |                       | will happen; however, latency issues might start |
|                              |                 |                       | to happen as soon as the VM is resized, and might|
|                              |                 |                       | be hard for operators to identify the root cause.|

| cold migration               | false           | no                    | Same as resize                                   |
| cold migration               | true            | no                    | Same as resize                                   |

| cold migration               | false           | yes                   | Same as resize                                   |
| cold migration               | true            | yes                   | Same as resize                                   |

| live migration               | false           | no                    | Same as resize, but it is a live migration       |
| live migration               | true            | no                    | Same as resize, but it is a live migration       |

| live migration               | false           | yes                   | Same as resize, but it is a live migration       |
| live migration               | true            | yes                   | Same as resize, but it is a live migration       |