Have you ever heard about cloud computing? It is a hot topic in the IT industry and claims that you can allocate nearly unlimited computer resources on a pay-as-you-go basis. Are you not curious to know how they are able to provide such a service? The underlying technology that allows them to be able to provide such a service is hardware virtualization. If you don't understand how virtualization works, it could be difficult for you to imagine how they can add or remove resources instantly. Let's talk about how server virtualization works and what it offers us.

Why should we use server virtualization? Since the new generation of server class machines are becoming much more powerful, it becomes quite difficult to use up all the system resources if we only install one Operating System (OS) on it. Also, renting multiple server racks in a data center is expensive. Centralizing multiple servers into a limited set of powerful servers seems to be a more cost-effective solution, thus virtualization appears. Server virtualization allows users to create multiple system objects called virtual machines (VMs) that act like normal computers. Virtualization of physical devices implies that each virtual machine has its own CPU units (called vCPU), memory, hard disk, and network card, according to a user's allocation. When the user turns on the virtual machines, different OSes can be installed on them. Therefore, better system resources utilization can be achieved. The following figure shows the difference between a physical machine and a virtual machine:

Depending on the kind of processor used, there are three different types of virtualizations available: full virtualization, para-virtualization, and hardware-assisted virtualization. In order to state the difference between these virtualization methods, we need to know how a Control Processing Unit (CPU) executes code that a user has passed.

During any process execution, a CPU is the computing unit that executes predefined instruction sets to generate the results that the program had defined. But, it is dangerous if we give full access to all applications on our devices. For example, if there is no restriction on hardware access, a web page will be able to inject suspicious code into local memory and it may further damage the data stored in our computers. Therefore, a term called protection ring ranking, which ranges from 0 to 3 under the x86 architecture is used to protect our hardware. Normally, Ring 0 (also called supervisor mode) is used in the OS to monitor and control system resources. Ring 3 (also called user mode) is used for a user application, and if we would like to have access to hardware, a system call provided from the supervisor mode must be made. The following figure shows the system ring structure for the x86 platform:

Based on the levels of virtualization, we have the following different types of virtualization types: full virtualization, para-virtualization, and hardware-assisted virtualization:

So, when both physical and virtualized platforms are running operating systems, how can we distinguish them from each other? Here come the terms: host and guest, explained in the following points:

We have discussed why we need to learn server virtualization and how virtualization works, so are you curious about how many types of major virtualization software are on the market? What are the differences between them? Let's take a deep look at it:

After we have talked about so many concepts, let's learn how to configure Proxmox! I assume you have downloaded and installed your own Proxmox. If you face any difficulty during the installation, you can refer to the following link:

https://pve.proxmox.com/wiki/Quick_installation

The following steps will help you perform basic administration in Proxmox VE:

Creating a kernel-based virtual machine

We have seen how to create OpenVZ-based virtual machines; now, let's learn how we can create a kernel-based virtual machine:

1. Click on the Create VM button in the top-right corner, as shown in the following screenshot:
   
2. Specify a name for identification, but please note that the name is not directly applied to the VM's hostname, as shown in the following screenshot:
   
3. Then, we need to choose the type of platform we are going to create, shown as follows:
   
4. Next, the OS installation source is specified, shown as follows:
   
5. After that, we have to allocate the hard disk for the virtual machine. Pay attention to the hard disk's Format you have chosen; the raw format will instantly allocate the space you have defined (that is, if you defined a 20 GB hard disk, a disk image with 20 GB will be created under the host server) while the QEMU format will only allocate the space based on the current usage of the guest OS. If the guest OS takes up 8 GB, then a disk image with 8 GB in size will be created. For more details, refer to the Virtual disk options under Proxmox VE section. The hard disk window is shown in the following screenshot:
   
6. The number of cores/sockets and the CPU types have to be specified for the guest, as shown in the following screenshot:
   
7. Of course, we need to allocate the amount of memory for the guest platform, shown as follows:
   
8. For the network part, similar to the OpenVZ creation, we have bridged mode and NAT mode. Here, we can specify Model for the virtualized VM; generally, choosing Intel E1000 and Realtek RTL8139 should be fine. The VirtIO option provides better performance, but the driver must be installed on the guest OS before it can be used. This is shown in the following screenshot:
   
9. A summary of the configurations is displayed for confirmation.
10. Just as it appears for OpenVZ, we can find our created VM under the menu, as follows:
    

Isn't it easy to create a virtual machine with a web interface? By the way, you can identify whether a virtual machine is an OpenVZ container or KVM by its associated icons, as shown in the following screenshot:

There are two types of virtualizations available in Proxmox: OpenVZ and KVM. What are the differences between them?

OpenVZ is an operating-system-level virtualization based on the GNU/Linux kernel and the host operation system. Theoretically, OpenVZ is not a type of virtualization but more like the jail concept in Linux. Since a patched Linux kernel is needed, only Linux guests can be created. All guests are called containers that share the same kernel and architecture as long as the host OS, while each container reserves a separate user space.

There is no overhead for OpenVZ as containers can call hardware resources directly. However, since all containers share the system kernel of the host OS, a system-related problem might appear during the host OS kernel upgrade. Besides, OpenVZ stores container files as normal files in the host OS, so it is not recommended to use OpenVZ if there are confidential files stored in the virtual machine. Kernel-based Virtual Machine (KVM) is basically a hardware-assisted virtualization with the modified Linux kernel built with the KVM module. KVM itself does not perform any emulation or virtualization. Instead, it simply exposes the /dev/kvm interface. QEMU is chosen as a software-based emulator to simulate hardware for the virtualized environment. The structure of KVM is shown as follows:

As we can see, overheads on frequent requests appear in QEMU-emulated devices. Thus, an improved version for KVM is published with VirtIO drivers. VirtIO creates a buffer for both the guest system and QEMU, which speeds up the I/O performance and reduces the overhead. To enjoy the performance burst, a VirtIO driver must be installed separately on each emulated hardware device. In the following diagram, we have demonstrated the new structure of the KVM machines with the VirtIO drivers installed:

The following table shows the supported operating systems provided by OpenVZ and KVM:

During the virtual machine creation, the following virtual disk options are available:

Since the OpenVZ filesystem is only file-based, it is possible to pack the configurations and the disk image into a file for further deployment. We can create our own template file (which will be introduced in Chapter 7, Disaster Recovery on a Proxmox VE Cluster), or download the template file (also called a virtual appliance) via a web interface or from http://download.proxmox.com/appliances/.

Use these steps to download an OpenVZ template:

In this chapter, we introduced the different modes of server virtualization, the reasons for using it, and how it actually works. We also caught a glimpse of the development on server virtualization by knowing the background technologies. Besides that, we learned the features and strengths of the different hypervisors on the market. Moreover, we went through some basic administration techniques of Proxmox, including the creation of virtual machines, importing OpenVZ templates, and viewing a guest system via the web console. Most importantly, we learned how to distinguish KVM and OpenVZ, which affects our decision during VM creation, for example, security versus performance.

In the next chapter, we are going to learn some concepts on high availability and the introduction of the Proxmox cluster.