Originally Posted by satimis
I only have 2 servers (VM-1 and VM-2) for the redundancy setup. Whether I must install 4 servers (4 VMs) to proceed.
To follow the method in the howto, you need 4 nodes (servers). You could get away with 3 if you didn't want redundancy for the load balancer.
In case of only 2 servers (2 VMs) are required for my case whether I can clone Server-1(VM-1) as Server-2(VM-2)? OR Server-2(VM-2) must be a base server without Apache, MySQL, PHP, etc. installed?
You can clone your completed server (obviously changing the necessary stuff, including your ssh private key).
The idea behind the howto is to have multiple cheap, replaceable web hosts at the back end. All run the same apps and have the same data (either replicated, or via a backend shared filesystem). In times of high load, you can easily deploy more nodes to add capacity, and redeploy them in quiet times. This is a great way to do it, as the nodes can be relatively inexpensive and as I say, it's easy to add capacity. The other way of having a highly available server is to make a failover cluster with shared filesystems. All the apps run on one big server, and in the case of failure, the disk is remounted on the standby server and it takes over. The downside is that you have a server sitting and waiting, generally doing nothing, and the only way to add capacity is to buy a bigger server (generally all servers need to be upgraded as they all need to be able to run the load by themselves).
You have a load balancer at the front end to direct traffic amongst the web servers. The howto also describes a method of providing a failover capability for the load balancer in case of failure or planned maintenance of the primary load balancer.
Hopefully that makes some sense. Have fun with your project. HA is an interesting area of system administration.