Using the most recent Nova Essex builds on Fedora. It appears that Nova isn't fully deallocating fixed_ips:
After deleting an instance I'm seeing the following warnings in network.log:
2012-03-13 04:29:17 WARNING nova.network.manager [req-4c766b6c-7e2f-4ce1-a021-4df6b75aeb48 None None] IP |192.168.0.2| leased that isn't allocated
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The following error from nova-manage (I fixed this display issue in other bug):
[root@nova1 ~]# nova-manage fixed list
network IP address hostname host
192.168.0.0/24 192.168.0.0 None None
192.168.0.0/24 192.168.0.1 None None
Command failed, please check log for more info
2012-03-13 04:34:43 CRITICAL nova [req-4f51bcdf-0f87-4e50-a521-56536e24fbe2 None None] 2L
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Here is the fixed IPs table:
mysql> select * from fixed_ips where instance_id is not null;
+---------------------+---------------------+------------+---------+----+-------------+------------+-------------+-----------+--------+----------+----------------------+------+
| created_at | updated_at | deleted_at | deleted | id | address | network_id | instance_id | allocated | leased | reserved | virtual_interface_id | host |
+---------------------+---------------------+------------+---------+----+-------------+------------+-------------+-----------+--------+----------+----------------------+------+
| 2012-03-13 04:10:37 | 2012-03-13 04:33:20 | NULL | 0 | 3 | 192.168.0.2 | 1 | 2 | 0 | 1 | 0 | NULL | NULL |
+---------------------+---------------------+------------+---------+----+-------------+------------+-------------+-----------+--------+----------+----------------------+------+
1 row in set (0.00 sec)
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Nova will work fine until an IP gets recycled at which point things go downhill.
I'm assuming you can't use force_dhcp_release, but i'm curious why _disassociate_