Hello,
we are having some "problems" with Org vDCs which are deployed under the allocation pool model. It is not possible to use the allocated RAM by 100% (or any value close to 100%). When trying to start VMs we get a resource allocation error from vCenter (insufficient resources....). When looking at the resource pool it turns out, that "Used reservation" is larger than the sum of reservation of the already running vms.
Example
Org vDC has 13GB RAM allocated with 20% guarantee = Resource Pool gets 2662 MB of reservation
VM1 is configured with 5GB RAM = gets Reservation of 1024MB
VM2 is configured with 6GB RAM = gets Reservation of 1229MB
Sum of VM reservations = 2253MB
From the customers point of view it looks as there are still 2GB of RAM available, from a (calculated) RP view there are still 409 MB available which are 20% of 2 GB RAM. So the customer configures a new VM with 2GB, tries to start it, but it is not working because of insufficient resources.
When looking at the resource pool in detail it is somehow clear why it fails. The used reservation is 2521 MB, so only 141 MB are available. Thats about 700 MB usable for the customer.
I believe that this has to do with the overhead of the VMs, although the resource pools overhead value does not explain that difference
That also happens when using other percentages of guaranteed RAM.
That behaviour is - although somehow understandable - annoying.
To avoid support calls from customers complaining about that issue there are 2 (in my opinion) possibilities
1. Explain customer the situation/reason and tell them to order more RAM. First of all i don't believe that the majority will understand it or cares. And as long as we can't calculate how much more is needed it is a no go.
2. Add a buffer to the ordered amount of RAM. That is what we do right now - we add 10%. It works, but the customers see that they have more RAM available than they ordered. Integration with billing is difficult, too.
Are other users of vCloud Director aware of that issue, too ? Is ist planned to "fix" that in future releases ? Like vCloud Director auto adjusting RP reservation limit somwhow.
Best regards
Carsten