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 Thank you both Melanie and Matt for trying to assist me.

 I have double checked the nova.conf at the controller and here is what 
 I have (ignored hashed lines and obfuscating sensitive data): 
 https://pastebin.com/hW1PE4U7

 As you can see I have everything with default values as discussed 
 before with Melanie except the filters and the weight that I have 
 applied that should lead to VM stacking instead of spreading.

 My case scenario is with two compute hosts (let's call them "cpu1" and 
 "cpu2") and when an instance is already placed on "cpu2" I expect the 
 next instance to be placed also there. But instead is placed on "cpu1" 
 as you can see from the scheduler log that can find here: 
 https://pastebin.com/sCzB9L2e

 Do you see something strange that I fail to recognize?

 Best regards,

 G.

> On 4/16/2019 7:03 PM, melanie witt wrote:
>> To debug further, you should set debug to True in the nova.conf on 
>> your scheduler host and look for which filter is removing the desired 
>> host for the second VM. You can find where to start by looking for a 
>> message like, "Starting with N host(s)". If you have two hosts with 
>> enough RAM, you should see "Starting with 2 host(s)" and then look for 
>> the log message where it says "Filter returned 1 host(s)" and that 
>> will be the filter that is removing the desired host. Once you know 
>> which filter is removing it, you can debug further.
>
> If the other host isn't getting filtered out, it could be the
> weighers that aren't prioritizing the host you expect, but debug logs
> should dump the weighed hosts as well which might give a clue.