http://www.vpsee.com/2011/11/how-to-delete-a-openstack-nova-zombie-instance/
前天强制重启一台 OpenStack Nova 控制结点以后发现虚拟机消失，但是 euca-describe-instances 命令显示 instances 仍然是 running 的状态，使用 euca-terminate-instances 终止命令仍然无效，暂时把这样的 instance 称作"僵尸实例（zombie instance）"：
# virsh list
Id Name State
----------------------------------
# euca-describe-instances
RESERVATION r-bkl83j20 bangcloud default
INSTANCE i-0000001d ami-00000002 172.16.39.121 172.16.39.121 running vpsee (vpseecloud, node00) 0 2011-11-10T12:45:12Z nova aki-00000001 ami-00000000
RESERVATION r-j335q6ny bangcloud default
INSTANCE i-0000001e ami-00000002 172.16.39.122 172.16.39.122 running vpsee (vpseecloud, node00) 0 2011-11-10T12:54:27Z nova aki-00000001 ami-00000000
# euca-terminate-instances i-0000001d
# euca-terminate-instances i-0000001e
和 删除 OpenStack Nova Volume 时遇到的 error_deleting 问题 这篇文章提到的解决办法一样，直接操作数据库来删除这2条僵尸实例的记录。登录 mysql，使用 nova 数据库，找出要删除 instance 的 id，然后删除：
# mysql -u root -p
Enter password:
mysql> use nova;
mysql> select * from instances;
mysql> delete from instances where id = ‘29‘;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`nova`.`virtual_interfaces`, CONSTRAINT `virtual_interfaces_ibfk_1` FOREIGN KEY (`instance_id`) REFERENCES `instances` (`id`))
MySQL 删除 id 为 29 的 instance 时触发外键限制错误，简单的办法是暂时关闭外键检查，等删除后再打开：
mysql> SET FOREIGN_KEY_CHECKS=0;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from instances where id = ‘29‘;
Query OK, 1 row affected (0.04 sec)
mysql> delete from instances where id = ‘30‘;
Query OK, 1 row affected (0.04 sec)
mysql> SET FOREIGN_KEY_CHECKS=1;
Query OK, 0 rows affected (0.00 sec)
删除 instance 29 和 30后再用 euca-describe-instances 命令验证一下：
# euca-describe-instances