fencing is a very important concept in cluster. not sure I can explain clearly in a few words. any how , forget about RAC first, just look at generic share-disk cluster issue.

let's say the cluster only has two nodes. both share the disk. ie. they both have the right to write to the same disk.

so this share access must be coordinated.

however , if for some reason , this coordination lost, it can be any reason, communication process hanging, you unplug the network cable between them, etc etc

at this moment both nodes are functioning normally except the lost coordination between them.

then the cluster has two problems to solve:
1. who will be the new member of the new incarnation of cluster , in this case, this is split brain issue.

2. once we decide who will remain in the cluster and who will go , how can we prevent the going node NOT to do something harmful to the cluster ? this is fencing issue. Bear in mind , the going node is working perfectly normal except the coordination part, so it still can write to the shared-disk.

There are a couple of approaches in fencing:

1.server fencing , in cluster terms, Shoot The Other Node In The Head (STONITH) , i.e the good node kill the going node.

( the way Oracle do is : reboot itself, once go through the reboot process, it just do the rejoin cluster again .
then the cluster can decide whether accept it or not )

2. I/O fencing. rather than trying to kill the node, it is working on the disks side to block the going node's access to disk, Sun , Veritas has solution on this way.

HTH. also try to do a google on terms like "fencing" , "split brain", "amnesia".

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dpjude:


我觉得不是会不会产生节点不一致的问题，而是由于某种原因导致一个或n个节点与其他n个节点不一致了，才发生brain split。
比如网络问题（interconnect断了）；时间不一致；misscount超时 等等。

而此时为保护整个集群不受有问题的节点影响，而发生brain split（或叫fencing？）

oracle采用的是server fencing，就是重启有问题的节点，试图修复问题。当然有很多问题是不能自动修复的。比如时间不一致，而又没有ntp；网线坏了。。。这些都需要人工介入修复问题。而此时的表现就是有问题的节点反复重启。