1. A method for predicting an availability of a high availability cluster, the method comprising: calculating, by a computer of a cluster system, a basic survival probability that the other node survives until a failure on one node of two nodes constituting a cluster is fixed; and determining, by the computer of the cluster system, an optimal number of nodes meeting a preset reference availability probability by calculating an availability probability for a predetermined range of the number of nodes on the basis of the basic survival probability, wherein the availability probability, which is a probability that other nodes survive until a failure on one node of entire nodes is fixed, is calculated by the following equation: P n = P 1 n - 1 + ∑ k = Qn n - 2 ⁢ P 1 k ⁡ ( 1 - P 1 ) n - k - 1 ⁢ ( n - 1 ) ! ( n - k - 1 ) ! Q n = [ n + 1 2 ] where P n is the availability probability, Q n is a quorum, P 1 is the basic survival probability, k indicates a quorum number, n indicates a node number, and n is an integer more than 1.