Code review comment for lp:~soren/nova/iptables-concurrency

I tried spawning > 20 instances (hypervisor KVM) at the same time on one nova-compute node and I got 2 failed instances. I think the problem is, that there are only 15 (or 16?) available NBD devices at the same time.

(nova.compute.manager): TRACE: Error: No free nbd devices

So there has to be a hard limit somewhere while using KVM as hypervisor. Is there a flag to limit the number of spawning instances on one node at the same time? If not we should introduce one. Or is it possible to use more than 15 NBD devices at the same time?

I tested the code against rev 713 and I don't get the error described in #722477. So it seems working for me :) (but i didn't read the code)