一段关于fencing的阐述，个人觉得很精辟（zt）

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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