I am newer to VCS, Can someone please help me about why we are using 3 coordinater disks why we wont use less than 3 coordinator disk to configure IO fencing in VCS?
the reason of by default needing 3 fecing devices is for fencing mechnism to work reliablly in a true slit brain scenario.
When there is potencially a split brain scenario, to save guard usder data, each node in the cluster will race to gain the majority of fencing disks - whichever node gains the majority of fencing disks will stay in the cluster and the node fails in ganing the majority will be fenced out from the cluster (losing access to the shared storage) thus preventing data corruption.
If there was only two fencing disks and each node wins one disk, then will would not be a facing race winner. If there was only one disk for fencing, then the node fails in fencing race may only because of disk I/O issue, not truely a network partition related split brain. Therefore for fencing mechanism to work reliabley,an odd number of fencing disks (3, 5 or 7) is a must.
At one stage this requirement was dropped to minimum odd number 1 howevrr there were some issues with this implementation and this minimun number of fencinng device was retsored to 3 in current releases.
You may also consultwith the info in this technote https://www.veritas.com/support/en_US/doc/129683742-129683745-0/uxrt-731_v24119925-129683745 for more fencing related info.