This morning I was setting up some test instances on my home OpenStack deployment and ran into some issues. The first instance launched okay, but the second failed, telling me that no suitable host was found.

I have not really oversubscribed, or so I thought. Turns out I was oversubscribed on both RAM and Disk.

OpenStack has defaults surrounding this:
* Disk is not oversubscribed by default. There is a 1-1 subscription per host. In a production setup, this should never be changed – an instance launching on a host that could run out of space and take down all the instances on that node is definitely a bad thing.
* Memory is oversubscribed 1.5 times.
* CPU is oversubscribed 16 times (cores).

Disk and memory allocation are not so much as critical on standalone setups. These will inevitably need changing as more instances are stood up as OpenStack still counts shut down instances towards the limit, even for RAM and CPU.

Changing the defaults

The defaults be changed: just edit your Nova configuration (ie: /etc/nova/nova.conf) and look for:
* disk_allocation_ratio for disk
* ram_allocation_ratio for Memory
* cpu_allocation_ratio for CPU.

Change these limits as you see fit and then just restart Nova (ie: openstack-service restart nova).

Reference: http://docs.openstack.org/developer/nova/devref/filter_scheduler.html