关于oraclerac节点的驱逐

我这里主要是想讲讲rac中关于node被驱逐进而导致reboot的情况。
首先说说rac中的心跳, 关于oracle clusterware中的心跳有两种，如下：
1. Disk heartbeat (voting device) - IOT
2. Network heartbeat (across the interconnect) - misscount
这里的disk hearbeat是指votedisk心跳，我们都知道votedisk是仲裁盘，
那么它到底有什么作用呢？Oracle文档有这样描述的：

votedisk：
The Voting Disk is used by the Oracle cluster manager in various layers.
The Node Monitor (NM)uses the Voting Disk for the Disk Hearbeat, which is
essential in the detection and resolution of cluster "split brain".
NM monitors the Voting Disk for other competing sub-clusters and uses it for the
eviction phase. Hence the availability from the Voting Disk is critical for the
operation of the Oracle Cluster Manager.
The shared volumes created for the OCR and the voting disk should be configured
using RAID to protect against media failure. This requires the use of an external
cluster volume manager, cluster file system, or storage hardware that provides
RAID protection.
Disk heartbeat：
Each node writes a disk heartbeat to each voting disk once per second。Each node
reads their kill block once per second, if the kill block is overwritten node commits
suicide.During reconfig (join or leave) CSSD monitors all nodes and determines whether
a node has a disk heartbeat, including those with no network heartbeat.If no disk
heartbeat within I/O timeout (MissCount during cluster reconfiguration) then node is declared as dead.
Voting disk needs to be mirrored, should it become unavailable, cluster will come down.
If an I/O error is reported immediately on access to the vote disk, we immediately mark the vote disk
as offline so it isn't at the party anymore. So now we have (in our case) just two voting disks available.
We do keep retrying access to that dead disk, and if it becomes available again and the data is
uncorrupted we mark it online again. If a second vote disk suffered an I/O error in the window
that the first disk was marked offline. So now we don't have quorum. Bang reboot.

网络心跳就不多说了，指的是rac私网心跳。如下一段摘自网络：

Voting files are used by CSS to ensure data integrity of the database by detecting and resolving network
problems that could lead to a split-brain, so must be accessible at all times. There are other techniques
used by other cluster managers, like quorum server, and quorum disks which function differently, but serve
the same purpose.
Note that a majority of vote disks, i.e. N/2 + 1, must be accessible by each node to ensure that all pairs
node have at least one voting file that they both see, which allows proper resolution of network issues;
this is to address the possible complaint that 2 voting files provide redundancy, so a third should not be
necessary.
During normal processing, each node writes a disk heartbeat once per second and also reads its kill block
once per second. When the kill block indicates that the node has been evicted, the node exits, causing a node
reboot.As long as we have enough voting disks online, the node can survive, but when the number of offline
voting disks is greater than or equal to the number of online voting disks, the Cluster Communication Service
daemon will fail resulting in a reboot. The rationale for this is that as long as each node is required to
have a majority of voting disks online, there is guaranteed to be one voting disk that both nodes in a 2
node pair can see.

上面提到了对于votedisk(仲裁盘)，当cluster中有节点出现故障时，offline的votedisk个数必须小于存活votedisk个数，
否则会导致存活的节点reboot。
下面再来说说几个重要的参数值.
misscount： 网络心跳可以丢失的次数(单位是秒)
不同平台和版本的misscount默认值是不一样的，详见如下表格：

OS	10g (R1 &R2)	11g
Linux	60	30
Unix	30	30
VMS	30	30
Windows	30	30

另外如果使用了第三方cluster软件时，那么misscount值默认即为600s，这里脑裂也是第三方cluster软件来完成的。
diskhearbeat即disktimeout，在10.2.0.1+版本以后(打了patch 4896338)默认值是200s。
disktimeout也简称为DTO，但是文档上又把DTO细分为两种，如下：
-- SDTO，是short disk time out的简称，即节点添加或删除时cluster需要进行reconfigure的时间。
-- LDTO，是指正常的rac操作中允许votedisk i/o完成超时的时间。
rebootime：在10g~11g中默认都为3s，即是rac出现脑裂或节点被驱逐的时候，该节点将会在rebootime时间内被重启。
文档中提到node的驱逐从10201版本以后，不在根据DTO来决定，而是基于disktimeout，
默认情况下，misscount的值小于disktimeout。
那么, 在什么情况下会导致node被驱逐呢？如下:
· Node is not pinging via the network heartbeat
· Node is not pinging the Voting disk
· Node is hung/busy and is unable to perform either of the earlier tasks
根据文档中的描述，翻译过来的node reboot条件表格：

Network Ping	Disk Ping	Reboot（是否重启）
在misscount值 内完成	在misscount值内完成	否
在misscount值内完成	Disk ping时间超过misscount值，但是小于disktimeout值。	否
在misscount值内完成	Disk ping时间超过disktimeout值	是
Network ping时间超过misscount值	在misscount值内完成	是

修改如上几个参数值的方法如下：
$ORA_CRS_HOME/bin/crsctl set css misscount
where is the maximum i/o latency to the voting disk +1 second
10.2.0.1+版本，如果应用了patch4896338，那么还需要有如下的操作:
$CRS_HOME/bin/crsctl set css reboottime [-force] ( is seconds)
$CRS_HOME/bin/crsctl set css disktimeout [-force] ( is seco
最后需要说明一下的是，在使用了第三方集群如cluster软件以后，oracle就不再推荐修改misscount，
可以引发潜在的错误。oracle是这样解释的：
Do not change default misscount values if you are running Vendor Clusterware along with Oracle Clusterware.
The default values for misscount should not be changed when using vendor clusterware. Modifying misscount in
this environment may cause clusterwide outages and potential corruptions.
如上的信息大家可以参考如下的几个mos文档：
10g RAC- Steps To Increase CSS Misscount- Reboottime and Disktimeout
CSS Timeout Computation in Oracle Clusterware
Reconfiguring the CSS disktimeout of 10gR2 Clusterware for Proper LUN Failover of the
Dell MD3000i iSCSI Storage [ID 462616.1]
How to start/stop the 10g CRS ClusterWare [ID 309542.1]