5.3. 做一次失效备援

作为一个高可用的集群，我们在继续本文档之前，我们要需要测试失效备援。

首先，找到IP资源现在在哪个节点上运行。

# crm resource status ClusterIP
resource ClusterIP is running on: pcmk-1
#

Shut down Pacemaker and Corosync on that machine.

# ssh pcmk-1 -- /etc/init.d/pacemaker stop
Signaling Pacemaker Cluster Manager to terminate: [ OK ]
Waiting for cluster services to unload:. [ OK ]
# ssh pcmk-1 -- /etc/init.d/corosync stop
Stopping Corosync Cluster Engine (corosync): [ OK ]
Waiting for services to unload: [ OK ]
#

当Corosync停止运行以后，我们到另外一个节点用crm_mon来检查集群状态.

# crm_mon
============
Last updated: Fri Aug 28 15:27:35 2009
Stack: openais
Current DC: pcmk-2 - partition WITHOUT quorum
Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
2 Nodes configured, 2 expected votes
1 Resources configured.
============

Online: [ pcmk-2 ]OFFLINE: [ pcmk-1 ]

关于集群状态，我们有三个地方需要注意，首先，如我们所料pcmk-1已经下线了，然而我们发现ClusterIP不在任何地方运行！

5.3.1. 法定人数和双节点集群

This is because the cluster no longer has quorum, as can be seen by the text "partition WITHOUT quorum" (emphasised green) in the output above. In order to reduce the possibility of data corruption, Pacemaker’s default behavior is to stop all resources if the cluster does not have quorum.

当有半数以上的节点在线时，这个集群就认为自己拥有法定人数了，是“合法”的，换而言之就是下面的公式:

total_nodes < 2 * active_nodes

Therefore a two-node cluster only has quorum when both nodes are running, which is no longer the case for our cluster. This would normally make the creation of a two-node cluster pointless ^[14] , however it is possible to control how Pacemaker behaves when quorum is lost. In particular, we can tell the cluster to simply ignore quorum altogether.

# crm configure property no-quorum-policy=ignore
# crm configure show
node pcmk-1
node pcmk-2
primitive ClusterIP ocf:heartbeat:IPaddr2 \
    params ip="192.168.122.101" cidr_netmask="32" \
    op monitor interval="30s"
property $id="cib-bootstrap-options" \
    dc-version="1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f" \
    cluster-infrastructure="openais" \
    expected-quorum-votes="2" \
    stonith-enabled="false" \
    no-quorum-policy="ignore"

过了一会，集群会在剩下的那个节点上启动这个IP。请注意集群现在依然没有达到法定人数。

# crm_mon
============
Last updated: Fri Aug 28 15:30:18 2009
Stack: openais
Current DC: pcmk-2 - partition WITHOUT quorum
Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
2 Nodes configured, 2 expected votes
1 Resources configured.
============
Online: [ pcmk-2 ]
OFFLINE: [ pcmk-1 ]
ClusterIP (ocf::heartbeat:IPaddr): Started pcmk-2

现在模拟节点恢复，我们启动 pcmk-1 上面的Corosync服务，然后检查集群状态。

# /etc/init.d/corosync start
Starting Corosync Cluster Engine (corosync): [ OK ]
# /etc/init.d/pacemaker start
Starting Pacemaker Cluster Manager: [ OK ]# crm_mon
============
Last updated: Fri Aug 28 15:32:13 2009
Stack: openais
Current DC: pcmk-2 - partition with quorum
Version: 1.1.5-bdd89e69ba545404d02445be1f3d72e6a203ba2f
2 Nodes configured, 2 expected votes
1 Resources configured.
============
Online: [ pcmk-1 pcmk-2 ]

ClusterIP    (ocf::heartbeat:IPaddr):    Started pcmk-1

现在我们可以看到让某些人惊奇的事情，IP资源回到原来那个节点(pcmk-1)上去了。

^[14] Actually some would argue that two-node clusters are always pointless, but that is an argument for another time