Why odd number of voting disk

Simple Majority Rule, According to Oracle – “An absolute majority of voting disks configured (more than half) must be available and responsive at all times for Oracle Clusterware to operate.” Which means to survive from loss of ‘N’ voting disks, you must configure atleast ‘2N+1′ voting disks.

OR we can say a node must be able to see trun(n/2 + 1) voting disks to survive as a part of cluster . 

• When you have 1 voting disk and it goes bad, the cluster stops functioning.
• When you have 2 and 1 goes bad, as per simple majority rule cluster stop functioning 
• When you have 3 and 1 goes bad, the cluster runs fine . 
• When you have 3 and 2 go bad, the cluster stops .
• When you have 4 and 1 goes bad, cluster run fine . 
• When you have 4 and 2 go bad, the same, because the nodes can only access half, not > half.
• So you see 4 voting disks have the same fault tolerance as 3, but you waste 1 disk, without gaining anything.
• The recommendation for odd number of voting disks helps save a little on hardware requirement.